Bush party homicide trial: DNA expert examines machete mystery

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A lingering question has hung over the second-degree murder trial of Carlos Guerra Guerra: How did shooting victim Josue Silva’s DNA end up on the handle of a machete found at the bush party crime scene?

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The jury has been reminded many times over the seven weeks of trial that the profile of Silva, and not Guerra Guerra or others named in the case, was discovered after the machete was sent for testing.

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Forensic biologist Renata Dziak, who wrote the DNA report on the machete and a disposable mask found nearby, shed some light on the DNA discovery at the trial Thursday morning when she reviewed her findings for the jury.

Her answer was that there was no one definitive reason to explain the DNA finding, but several ways for DNA to be deposited on an object.

Guerra Guerra, 23, has pleaded not guilty to second-degree murder of Silva, 18, a Western University student who died of a gunshot wound to the abdomen, and not guilty to assault with a weapon of Silva’s best friend, Logan Marshall, at the southwest London bush party on July 31, 2021.

He is the sole defendant left in the trial after Superior Court Justice Patricia Moore reported that Emily Altmann, 22, and her defence team had been excused from the trial and that the jury was not to speculate why.

Silva died at the end of what had been a large gathering of more than 100 young people in a clearing at a wooded area on Pack Road near Grand Oak Cross. The jury has heard that an argument over a spilled drink led to Guerra Guerra being summoned to the party to help settle the score.

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The jury has already heard that Dylan Schaap, who arrived at the party with Guerra Guerra, has pleaded guilty to manslaughter.

And, earlier this week, Guerra Guerra’s defence lawyer, Ricardo Golec, told the jury it was admitted that Guerra Guerra discharged the firearm that shot and killed Silva and that the gun was brought to the party by Guerra Guerra’s group.

But the machete found at the crime scene remains a question mark. The jury has also heard Silva did own a machete that his friends saw at his home. A number of witnesses who saw the physical confrontation with Guerra Guerra and Schaap have testified they did not see any weapons on Silva and Marshall.

The machete, one of two identified in the case and made an exhibit at the trial earlier, was found near where Silva was shot. Dziak said under questions from assistant Crown attorney Kristina Mildred that the initial examination of the knife was to look for blood and test it for DNA.

However, even after using a stereo microscope, “blood was not detected on the machete,” she said. She added she could not conclusively say there was no blood on it.

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There could be traces of blood that were masked by materials like dirt or rust, but she added that the microscope should be able to pick up blood.

A DNA profile could also be from an assortment of other sources such as saliva, nasal secretions or skin cells.

The entire handle was swabbed for DNA analysis, which resulted in a profile with a mixture of DNA from three individuals. She added that the amount detected was 2.5 nanograms – and to illustrate how tiny the amount was, she told the jury to imagine taking a chocolate Smarties candy and breaking it into one-billion pieces. One of those pieces is one nanogram.

“If you look at the quantity of DNA on the handle, it’s not a lot,” she said during cross examination by Golec.

The DNA profile that represented 63 per cent of the mixture was, statistically, a high likelihood of coming from Silva. The chances of someone unrelated to Silva with the same profile were estimated at 150 million to one.

The other two profiles were from unknown sources and Guerra Guerra, Schaap and Marshall were excluded as contributors.

Dziak explained that there were different ways the DNA could be deposited on the handle. There could be direct contact by the source, or indirect contact where the DNA is transferred from an individual or from the object touching a surface that has the DNA on it.

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There is no scientific way to determine how the DNA was deposited or when, she said.

Dziak didn’t rule out Golec’s hypothetical that the machete could have been handled days or weeks earlier by a group of friends playing with the knife in a garage.

But she also wouldn’t rule out that the DNA could be transferred if the machete landed on grass where there was a bodily fluid.

Dziak also reported her findings from a disposable mask that was found at the crime scene. There was a mixture of two DNA profiles, with Guerra Guerra as the larger contributor.

Dziak said 93 per cent of the sample belonged to one individual and the chances of the DNA coming from another source were one trillion to one.

The trial continues Thursday afternoon.

jsims@postmedia.com

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