to have a contamination event," says public defender DNA specialist Bicka Barlow.
According to court transcripts, Gabriel, the crime lab's DNA unit supervisor, testified in December about two separate occasions of contamination. In a burglary case, in which evidence was tested "around 2009," the lab discovered the DNA of another worker, Tahnee Nelson, in a water control sample. The water control sample is a small vial of the water used in collecting a DNA sample off evidence to ensure the water itself doesn't contain DNA or other contaminants, Barlow says.
Nelson had screened the sample to determine what bodily fluids were
present and prepare it for DNA testing, but did not actually perform the DNA test.
Gabriel testified that he had spent "more than a few days" reviewing
"the overwhelming majority" of the 100
to 150 cases in which Nelson had tested DNA. Gabriel said he
verbally counseled Nelson for about a half an hour about why the tainted sample occurred, and reviewed the clean laboratory procedures with her that minimize the risk of contamination.
Yet when asked by defense attorney Erwin Fredrich if he
undertook an investigation to determine how Nelson's DNA got into the sample in the first place,
Gabriel replied, "There was no such investigation."
Fredrich later questioned Gabriel: "If you don't
know the cause, how can you be sure that you will not have further
contamination events?" (The prosecutor objected to the question on grounds of speculation, and Gabriel never answered.)
The hearing -- which was not conducted before a jury -- was part of the trial of Donzell Francis, who was
subsequently found guilty of assaulting a transgender prostitute. The crime lab had its own Francis snafu, earning media scrutiny by sitting on evidence for years. Yet the contamination discussed in the hearing occurred while analysts tested evidence in other, unrelated cases. Francis' counsel had hoped to bring up the incidents in trial to cast doubts upon the crime lab's work; the judge ruled they could not.
Gabriel testified about a second incident of contamination in 2009 during another burglary case. This time, a lab worker had found another
analyst's DNA profile in the water control sample. Again, Gabriel said
he reviewed the lab procedures with both analysts, and documented the
Gabriel testified he had not changed any of the lab's
procedures regarding contamination since the discoveries, but added they were the only such incidents he's aware of.
"...I have recorded two
contamination events in over 2,000 cases," he said. "That's what exists
in the laboratory, as far as I'm aware. We could speculate and do
scientific tests for academics' sake, and determine if there might be
one or two contamination events that went unnoticed. But, again, for the
purposes of this case, that's, like I just said, an academic study."
it could be done?" Fredrich asked.
wouldn't spend my time doing it," Gabriel answered.
Note: An earlier version of this story misidentified the man in the photo. He is San Francisco Crime Laboratory Director James Mudge. SF Weekly regrets the error.