During hours of dense testimony Tuesday, jurors in Matthew Farwell’s federal murder trial heard from a molecular geneticist who tried to undermine some of the DNA evidence they’d listened to earlier in the proceedings.
At one point, Judge Denise Casper had to tell the witness and counsel that jurors might not be following and they should try to get back on track.
The geneticist called by the defense, Maher Noureddine, took jurors through how DNA testing works and explained why some of the results analyzed in this case at the Mass State Police Crime Lab weren’t as accurate as they could have been.
Farwell, a former Stoughton Police Officer, is accused of killing Sandra Birchmore, 23, and her unborn child in February 2021.
According to prosecutors, Farwell strangled Birchmore to try to cover up alleged wrongdoing – namely that he started a sexual relationship with the victim when she was 15 – then staged her body to make it look like she’d hung herself from a closet doorknob in her Canton apartment.
But Farwell’s defense contends that Birchmore hung herself after Farwell told her he wasn’t the father of her baby and ended their relationship.
Witnesses for the prosecution testified earlier in the trial that Farwell’s DNA was found both in Birchmore’s underwear and on the ligature that was found wrapped around her neck.
Noureddine, who was asked by the defense to examine that work, took issue with how evidence had been handled.
The ligature was “crumpled into a big ball,” he said, which means that DNA could have transferred from one part of the nylon duffle bag strap to another.
When it came to analyzing the DNA found on the strap and the clothing, he said certain methods used by the MSP crime lab could have led to changes in their results.
Some of the samples were too complex, with at least four contributors, one of the matches to Farwell should have been noted as a partial profile, and Birchmore’s DNA was assumed in situations it shouldn’t have been, Noureddine said.
The way DNA mixtures are analyzed, Noureddine and government witnesses explained, analysts can only offer a probability that someone is a contributor. Those probabilities range and may have also changed had the lab compared the samples to Farwell’s brother William Farwell’s DNA, he said.
William Farwell, like his brother, is a former Stoughton cop who also had a sexual relationship with Birchmore. He testified under a grant of immunity last week.
On cross examination, Assistant United State Attorney Elizabeth Riley pushed back on some of the issues that Noureddine raised.
There was a moment, during a somewhat tense exchange about the work Riley said the MSP lab did to make the assumption about Birchmore’s DNA, when they started to discuss the manual validation the lab did versus the computation Noureddine said should have been done, that Judge Casper interrupted.
“Counsel and sir, you may be losing the jury here,” she said.
“You’re not saying that not assuming Sandra Birchmore’s DNA would have made Matthew Farwell’s DNA go from 6.2 trillion [likelihood] to exclusion, are you?” Riley asked.
“No,” Noureddine replied.
The geneticist also confirmed during cross that evidence supported the hypothesis that Farwell’s DNA was on the ligature and on the underwear.
But he couldn’t say when or how the DNA got there.
“It could stay there for decades… it could be removed or degraded within hours,” Noureddine testified. “That’s why you can’t use DNA evidence as a time clock for anything.”
At the end of the trial day, the defense also called forensic anthropologist Angi Christensen, who used to work for the FBI and analyzed another disputed piece of evidence in the case: Birchmore’s hyoid bone.
Jurors have heard from the government that the fracture in the small, floating neck bone means that Birchmore must have been strangled, but from the defense, there’s been testimony that the fracture alone can’t lead to any conclusions about the young woman’s cause of death.
Christensen only testified to analyzing a CT scan of the bone and confirming that it was fractured.
She said that according to literature those fractures are most common in strangulations, but they can happen during hangings. Christensen said that even though the bone is more flexible when it’s unfused, usually in younger people, there are still known instances of it breaking.
The defense is expected to rest their case tomorrow, with no proceedings held Thursday, and closing arguments coming likely on Friday.
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