Source: Rom J Leg Med 34: 53–60, 2026; DOI: 10.4323/rjlm.2026.53
Kristina Baumjohann, Mark Benecke
Abstract: German law allows for a particular type of scientific expert witness: Independent Certified and Sworn in Experts. Any bias would lead to unlimited liability for all financial or legal damage that may result from scientific inaccuracies. In contrast to laboratory reports, criminalistic thinking is often involved in our work, including interpretation of absent stain evidence. Here, we provide an examplary case report paid by the defendant: A son was accused of having stabbed his stepfather to death. When we compared the son’s statements with the stains, not only blood and DNA traces were significant but also stains that were absent (but would have been expected if the defendant’s statements had been true). The defendant’s statement did not match the stain evidence, including missing stains.
Keywords: independent certified and sworn in experts, German law, DNA, blood spatter, stain evidence, inclusions, exclusions.
INTRODUCTION
As a Independent Certified and Sworn in Expert Laboratory, our work must be objective by oath (Trade Regulations Law § 36; Law conc. Renumeration of Scientific and Other Experts, last changed 7. April 2025 and § 407a of the German Civil Law Procedure Code). In contrast to Anglo-American laws, work for one “side” is not excluded but any scientific inaccuracy would make us liable for all financial or legal damage resulting from a skewed decision of the court based on our statements. In contrast to regular technical work, criminalistic thinking is often involved in our work, including interpretation of absent stain evidence. In the following case, a young man was charged with killing his stepfather by inflicting two stab wounds to the chest. He stated that he was attacked by his stepfather with a knife in the entrance hall of their home. The stepson suffered four stab wounds to the upper body which reportedly bled immediately. Previously, the stepfather was said according to the prosection, to have exerted severe violence against his neck. The son allegedly managed to take the knife away from the stepfather and defended himself against him. The dispute shifted from the hallway to the living room. His mother testified that she had thrown chairs at her son in the living room because he attacked her husband (the stepfather) with the knife.
According to her, her husband was visibly bleeding at that time, but her son was not. The stepfather initially managed to escape through the adjoining kitchen into the garage, where he died. The son went up to the upper floor by the stairs after the altercation in the living/ dining area and went into the bathroom there.
According to his mother and his two stepbrothers, his shirt showed visible bloodstains only after he left the bathroom. After that, bleeding, the son entered further rooms on the upper floor (his room, hallway, parents’ bedroom), took the stairs to the ground floor, left the house through the front door, returned and sat on the living room sofa to wait for the ambulance. We were asked by the son’s attorney to compare his client’s statements with the sequence of events assumed by the court, working on the trace evidence.
We based our work on the critical review of the traces established and secured in the file (e.g. blood, DNA) and addressed only the traces relevant to our concerns. Both secured traces and non-existent traces – but expected depending on the description – were significant in this regard. This case report reflects problems arising from an experts stain lab. This is a profession reflected in a German law for highly specialized, independent, sworn in technical experts.
Court’s assumptions vs. son’s statements
We compared the court’s assumptions with the son’s statements and compared them with the biological traces. The (blood) traces relevant to our investigation could be attributed to the corresponding individuals through DNA analysis (Tables 1, 2).
We focused on criminalistic inclusion and exclusion procedures, and found the exclusions most relevant, too. Here, we describe the criminalistic application of our findings.
Attack by the stepfather in the entrance hall
Court: The son attacked his stepfather with a knife in the entrance area of the corridor.
Son: I was first attacked by my stepfather and defended myself against him with the same knife.
The five blood stains of the son in the entrance hall were created by the contact of a bloody surface with another surface (Figs. 1-5), by gravity induced dripping from a surface or object (Figs. 4, 5) as well as by active movements (without further acceleration) (Figs. 5, 6).
Figure 1. Sketch of the location of the stains in the hallway (S = Son).
It is unknown when the bloodstains appeared. Assuming that the son was not dripping blood after the confrontation, his blood must have been from at a later time, possibly when he left or returned to the house. We have no information as to whether the ambulance service could also have caused the traces of blood in the hallway.
The DNA swabs of the knife (Fig. 7) all showed the son’s DNA, while some swabs also contained the father’s DNA mixed in varying quantities (Tables 1, 2). The absence of the stepfather’s DNA on the knife handle and sheath supports the court’s assumption regarding the perpetrator role of the son: If the stepfather had held the knife handle, his genetic material could have been detected at least in small quantities. This assumption is supported by a study by [1]: There, the quantity of DNA transfer from a knife-wielding assailant to the knife handle was examined. If no DNA traces of a person are found on a knife handle, that person can be ruled out as the person wielding the knife. The person wielding the knife leaves the largest proportion of DNA on the knife handle.
Blood stains in the entrance hall as well as the DNA distribution on the knife supports the court’s assumption.
Alleged throttling by the stepfather
Court: In the altercation between the son and his stepfather, the latter did not exert strong force against the son’s neck.
Son: My stepfather allegedly tried to throttle me.
Figure 2. Contact stain pattern on the door frame of the basement door.
According to the son, the stepfather first allegedly grabbed the son by the front of his shirt collar which could be confirmed by the DNA results: While the rear collar area showed the son’s DNA, the front area also showed the stepfather’s DNA. On contact with objects, surfaces, skin, etc., genetic material is transferred to varying degrees [2-13]. In this context, it also depends on the exact detection method how much genetic material can be detected.
The son did not provide any information concerning the possible duration of the alleged throttling by his stepfather. From medico-legal perspective, there was no evidence for this as the son did not exhibit any strangulation marks, abrasions, or hemorrhages (so-called petechiae) that can occur with direct finger pressure or the strong pressure of the hand/hands on the skin of the neck [14]. Petechiae in the face or mucous membranes of the head are no longer considered as conclusive and/or sole indications of strangulation or suffocation [15]. No expected typical defensive injuries such as “scratches or abrasions on the backs of the hands, forearms, face, or upper back“ [14] were found in the son.
The deceased stepfather showed no hemorrhages in the tissue of the chest area that would indicate the forceful pushing of the son’s head against his chest – which the son claimed to have done after the alleged throttling. The stepfather’s palms showed no DNA of his stepson. If an attacker strongly strangles a victim, the palms of the attacker’s hands show DNA traces of the victim [16, 17]. According to the forensic medical report, the abrasion on the son’s neck also did not result from an attack on his neck (Fig. 8).
Result: The court’s assumption that the stepfather did not exert strong force on his stepson’s neck is reinforced by the traces (or lack there of) described here.
Chair throwing by stepfather
Figure 7. Knife with blood stains and knife casing.
Court: The mother threw chairs at her son to make him stop attacking the stepfather.
Son: My stepfather allegedly threw chairs at me.
A puncture mark on the backrest of a chair lying on the floor confirmed the mother’s account that she had thrown chairs at her son so that he would stop attacking her husband (Figs. 9, 10). Since the son was holding the knife at that time, this stab could have resulted from a defensive movement on his part. On the other hand, he had stated that his father had thrown chairs at him. No clear traces were found to either support or refute this.
Blood of the stepfather were recovered on the backrest of a chair (Fig. 9). Accelerated blood splatters from the then bleeding stepfather could have occurred, provided he had thrown chairs at his stepson, too. No dripping or accelerated bloodstains from the son were found in the living and dining area, despite the claim that he fended off at least two chairs there which his mother had thrown at him as he had already been “seriously injured” in the hallway earlier.
Result: There is no evidence to support the son’s statement.
Vase throwing by stepfather
Court: The stepfather, during his escape into the garage, knocked against a vase, which subsequently broke.
Son: My stepfather allegedly threw the vase at me.
Figure 9. Location of the stains in the living and dining room area (S = son, SF = step father).
Blood of the stepfather was recovered next to the dining table (Figs. 9, 11). His wife testified that her husband had taken refuge behind the dining room table in front of their son and held onto the back of a chair. She also stated that her husband, during his escape into the garage, bumped into a vase standing on the windowsill which then fell and broke. These fragments were located in the area of the original position of the vase and not in the area of the son. Among the fragments, a trail of blood was visible while the fragments themselves were not bloodstained (Fig. 12).
Result: This contradicts the statement of the son, the stepfather threw the vase at him as he had not been standing in the area of the fragments.
Self-infliction of stab wounds vs. infliction of stab wounds by stepfather
Court: The son did not bleed after the altercation and during the subsequent actions on the ground floor and caused the wounds himself in the upper bathroom.
Son: My stepfather inflicted four stab wounds on my own upper body, which immediately bled (visible through the clothing).
The areas of the stab wounds were accessible as well to the son himself as to an attacker standing opposite him (Fig. 13). While the right side of his chest shows superficial wounds like hesitant „test stab wounds“. The wounds on the left side of his upper body are significantly larger and deeper (especially on the lower left) and bled significantly more heavily. Assuming self-infliction, the right stitches could have been applied first, followed by the upper left stitch and finally the lower left stitch.
His shirt’s left front side is clearly more heavily soaked with blood than the right side of the shirt (Fig. 14) which indicates bleeding from the stab wounds but not the order in which they occurred. The first stab may have occurred at the upper wound on the left upper side of the shirt, as indicated by the barely visible bloodstains (Fig. 15).
This is in accordance with the presumed sequence of the stab wounds and could support the court’s assumption of self-infliction of the wounds by the son.
Figure 12. Blood stains (drops) from the stepfather on the floor and under the broken vase fragments (red circle).
Only blood traces of the son were found on the upper floor of the house (Fig. 16). No blood could be detected in the vicinity of the drain of the left washbasin, although the son stated washing blood from the knife and his hands in the left bathroom sink (Fig. 17). If the blood had been excessively diluted by water or was simply not present, could not be determined – nor could it be clarified whether the son had actually washed the blood off the knife and his hands there. The point in time at which the bathroom was used by the son cannot be determined from the presumed or absence of traces. He did not necessarily inflict the stab wounds in the bathroom upstairs althoug the mother and the two stepbrothers stated that his shirt showed bloodstains after leaving the bathroom.
The son’s blood was also found in the parent’s bedroom (Fig. 18); the circumstances under which the son lost blood there are unknown. He claimed to have opened only the bedroom door (not the balcony door) a little. Later, he stated that he had gone to the balcony door to make sure that his parents were not in the room.
Figure 15. Cuts through the son’s shirt.
This statement seems incomprehensible as he did not have to cross the room to ensure that his parents were not there.
In the son’s room, several drip traces and smear marks of his blood were found. A black jacket on his bed showed a bloodstain on the left inside. Allegedly, it should have originated from lying bleeding on the jacket which could explain the origin of this blood trail. The weapon was lying on the bedspread, covered with the pillow and part of the duvet (Fig. 19).
Court: The son inflicted the wounds on himself upstairs, which is why there are no bloodstains on the stairs.
Son: The bleeding wounds on my upper body are from my stepfather’s knife attack on the ground floor.
The absence of his blood in the stairwell area neither confirms the court’s assumptions nor refutes the son’s account: at the time the stairs were used (going up and down), the source of the bleeding might either not have been present yet or could have been covered with a hand, a towel, or something similar.
In this context, a blue towel from the sofa with blood of both men may have played a role by being taken from the upstairs bathroom and placed or pressed it on the bleeding wounds before returning down by the stairs (Fig. 20). The son may have cleaned the knife blade with this towel on which the stepfather’s blood may have been present (in smaller quantities).
The origin of the blue towel remained unclear.
The combination of the son’s bloodstains on the upper floor with the absence of his (blood) traces during the altercation in the living and dining room support the possibility of a self-inflicted injury. Where the possible self-administration may ultimately have taken place could not be determined.
Figure 16. Biological stains found in the upper floor (S = Son, SF = Stepfather).
Possible versions of the course of events
The stepfather allegedly attacked his stepson in the hallway with a knife, inflicting four stab wounds to his upper body. The son was then bleeding in the hallway, but not in the immediate vicinity of the actual combat scene. Despite the rapid movements of the knife, no highly accelerated traces of blood were produced in the attack area. According to the son, he had already been bleeding (dripping) in the hallway. However, there were no additional dripping or movement-related traces of his blood in the rest of the ground floor of the house.
The son may have inflicted the stab wounds on himself in the upstairs area. The existing and onexisting traces predominantly align with the court’s assumption that the stepfather did not stab his stepson in the ground floor hallway. As most of the son’s bloodstains were found on the upper floor, this supports the court’s view of his culpability.
DISCUSSION
Our analysis shows that not only the secured traces were important for addressing the questions at hand, but also those that were absent. The overall picture of the evidence – fragments of the vase, absence of the son’s bloodstains in the living and dining room, absence of the stepfather’s accelerated bloodstains in the living and dining room, absence of the stepfather’s genetic material on the neck of his stepson and on the handle of the weapon, absence of the son’s genetic material on the stepfather’s palms, and, insofar as these count as trace-related information, statements by the family members regarding the bloodstaining of the son’s shirt – do not contradict the court’s assumptions about the son’s culpability.
This case shows that under German Certified and Sworn in Expert law, objective, non-partisan statements are available irrespective of the „side“ that mandates the expert.
References
→ see .pdf