Which knife is the murder weapon? NFI provides decisive evidence in Swedish case
A dead woman lies in a pool of blood in the stairwell of a Swedish apartment block. She has five knife wounds in her chest, back and arm. The suspect is her husband, an Iraqi refugee. The evidence the Swedish police find against the suspect is mainly circumstantial, until the Netherlands Forensic Institute (NFI) conducts specialist analysis of the possible murder weapon on their behalf. Peter Zoon, expert on Microtraces and Materials, examined various types of knives sent by the Swedish authorities. He investigated the composition of the metal and the coating on the blades. Eventually, a red kitchen knife from the suspect’s drawer speeded up the murder case.
The NFI received the request from the Swedish forensic institute in February 2019, after which it landed on Peter Zoon’s desk.The Swedes sent two of the victim’s ribs and two blue knives. “We were asked to investigate the ribs for (micro)traces that could have come from the two knives. That’s all I knew about the case”, says Zoon looking back. The Swedish authorities approached the NFI due to the specialist MIT (Microanalysis of Invasive Traumas) research it conducts. How did the victim’s injuries come about and what caused them? Those are the key questions in MIT cases. The team, consisting of a forensic anthropologist, an expert in striations, impressions and shapes and an expert in microtraces, investigates fatal stabbings using a multidisciplinary approach.
Bone damage
“We first carefully cleaned the ribs we had been sent in order to remove all the soft tissue and leave only the bones”, explains Zoon. “We soon saw obvious damage to the bones. Around that damage, red/pink particles could be seen with the naked eye. From experience, I know that such particles can come from the coating of a red knife.” However, the knives that the Swedes had sent with the bones were not pink or red but blue. So that was very simple: neither knife was the murder weapon.
A month after the initial report, a follow-up request was received from Sweden. Again Zoon was sent two knives: this time, they were two pink knives, so they had potential. But not all red or pink knives are the same, observes Zoon. “The composition of the paint can tell you more about the knife. If we know the composition, we can search our paints database, which is full of all kinds of coatings: those used for knives, but also for crowbars, doors and cars.” All the NFI experts add their paint samples from cases to the database, and they also exchange them with other European forensic laboratories.
Paint composition
Zoon explains how he approached the Swedish case. “I first put the bone under the microscope and then, using a special needle, I recovered a minuscule paint particle that I could measure. I use infrared spectroscopy to look at the composition of the paint. Because the ingredients the paint is made up of can be distinctive. Roughly speaking, paint consist of binders, pigments and fillers, which you can compare.” Unfortunately, the pink knives submitted displayed no similarities with the microtraces from the rib.
A year after the murder, in February 2020, the expert was sent another knife – this time a red one, from the kitchen drawer of the suspect, the victim’s husband. “This time there was a match”, recalls Zoon. “Our analysis revealed that the paint from the knife sent to us corresponded to the paint from the victim’s rib. We quickly fed that back to the Swedes.”
Metal particles
But the NFI investigation didn’t end there. As well as the paint, the experts had also found metal particles in the victim’s bone, which they were able to examine using electron microscopy. This technique does not allow for a high degree of differentiation: for example, the expert can see from the composition that it contains stainless steel, but not much more than that. Zoon and his colleagues then used a small ceramic circular saw to cut into the rear of the rib and break it open in order to be able to see clearly exactly what was in the stab wound. In the bone, they found exceptionally large metal particles measuring 1.5 mm across.
“That offered us opportunities”, explains the expert. “We were able to analyse the larger metal particles more closely. As a result, I was able to say not only that they were made of stainless steel, I could also determine which trace elements they contained.” And they again corresponded to the knife: no fewer than six elements within the steel matched. Ultimately, Zoon reported a combined evidentiary value of 10,000 or more for the paint and steel traces. “That is a high evidentiary value”, he emphasises. “It means that the results of the investigation are far more consistent with this knife being the murder weapon than another random red knife.”
Added value of the NFI
In April 2023, Zoon was called as an expert witness before the Swedish court to explain his report. Because these types of analysis techniques are not used in Sweden. “A case like this clearly emphasises the added value of the NFI”, Zoon believes. “At the NFI, we have many different disciplines under one roof, which are able to conduct investigations together and complement one another, as in this case. That is our strength.”
Based on the NFI’s investigation, the Swedish court concluded that the wounds were very probably caused by the knife found in the suspect’s kitchen drawer or a knife that was made in the same production run. This prompted the judges to ask how many knives were produced in the same batch as the knife in the kitchen drawer. According to the verdict, it was established that the type of knife belonging to the suspect was not very common and was sold in only a few stores in Sweden. The number of knives that could have been involved in the murder was limited in part by the fact that they must have been manufactured in the same production run. With this in mind, the Swedish court ruled that the likelihood that the knife in the kitchen drawer was the knife used in the woman’s murder was compelling. “The knife speaks volumes”, ruled the court. “Apart from the young children, the suspect was the only person who had access to that knife.”
Key evidence
The Swedish public prosecutor has expressed his gratitude to the Dutch experts for their work, particularly because the evidence against the accused was otherwise largely circumstantial. He describes the NFI’s work as “the most important element of the evidence in this case”. Zoon is pleased that he was able to contribute to establishing the truth. “After examining a number of knives that were clearly not the murder weapon, we eventually did arrive at the red kitchen knife, with conclusive results. Without this evidence, this case would probably not have been solved.”
The suspect was recently sentenced to 16 years in jail.
