Keaton, in a telephone interview from ASCLD's office, says the agency was not pleased to learn that the sample switch took place after Mudge's indications to the contrary. "Obviously, that doesn't make us happy," he says. "If we felt that we had a consistent pattern of misrepresentation, we would find that extremely serious."

To date, SFPD officials say, they have not sought to determine which cases might have been affected by Nelson's error. According to Mudge's August 2009 letter to ASCLD, Nelson processed 33 cases in the last quarter of 2008.

Police say the switch was a minor accident that was quickly corrected. "It's like we're at a social party," said Capt. Donna Meixner, who was promoted in June and put in charge of the SFPD's Forensic Services Division. "You're Peter, I'm Donna. They mix up our name tags. But you're still Peter, and I'm still Donna."

Prosecutor Rockne Harmon, pictured during the O.J. Simpson trial, says he is troubled that DA Kamala Harris concealed his report criticizing the DNA lab’s supervisor.
AFP/Getty Images
Prosecutor Rockne Harmon, pictured during the O.J. Simpson trial, says he is troubled that DA Kamala Harris concealed his report criticizing the DNA lab’s supervisor.
Deputy Public Defender Bicka Barlow first heard of a DNA sample mixup in a homicide case through a whistleblower letter.
JP Dobrin
Deputy Public Defender Bicka Barlow first heard of a DNA sample mixup in a homicide case through a whistleblower letter.

Despite the fact that records of the mistake by Nelson were destroyed, an internal-affairs investigation into the incident showed that there was "absolutely no coverup in this case," according to SFPD spokeswoman Lt. Lyn Tomioka. She noted that ASCLD ultimately chose to reaccredit the crime lab in October, its findings notwithstanding.

Tomioka acknowledged that the initial denials of the lab mishap "looked bad" in light of investigators' later findings, but said the lab has made great strides since then in increasing the transparency of its operations. "We have moved so far forward with the lab that this would be like churning up old history," she said, noting that the SFPD had promptly provided ASCLD's most recent findings to SF Weekly when asked.

Nelson, approached by a reporter after testifying at a recent court hearing, declined to comment on the sample switch. "I'm not at liberty to discuss it with you," she said. Mudge, who has left his position at the crime lab but still works at the SFPD, could not be reached for comment.

Gabriel, who earlier this year departed the lab for a position at Applied Biosystems, a genetic-testing products firm in Foster City, first learned of the ASCLD report from SF Weekly and initially said he might be willing to discuss it. After receiving the report from a reporter, he did not respond to further requests for comment.

Marc Taylor, president of Technical Associates, Inc., a private crime lab in Ventura, says DNA sample mixups aren't unheard of in the forensics business. What makes the chain of events in San Francisco so disturbing, he says, are the steps that were taken to conceal it — first the relabeling of test tubes and destruction of lab records, then Mudge's letter flatly denying any knowledge of the incident. These, more than the switch itself, are symptomatic of a lab that has no place handling evidence in criminal cases, Taylor says.

"For every one of those things that are discovered, there are dozens or hundreds that could have happened. That could result in a false conviction of somebody," he says. "If you're going to be switching things, if you're going to be hiding things, if you're going to be saying things didn't happen, you've lost your credibility. That laboratory should be shut down."

While the idea of a switched DNA sample in a homicide case is titillating stuff, it wasn't the first time the lab's workers had faced accusations of unethical conduct. In fact, at the beginning of 2008, another DNA analyst would make statements under oath before a grand jury that would later give rise to a complaint to Gascón — and yet another internal-affairs investigation.

In that case, criminalist Cherisse Boland gave testimony implicating two young men in a vicious murder. Yet they would later be acquitted, and troubling indications would surface that both Boland and the SFPD had ignored evidence that could lead to the real killer.

On the morning of Sept. 2, 2007, Byron Smith was gunned down in a stranger's garage on Velasco Avenue after fleeing from two men on bicycles. In the aftermath of the murder, police inspectors arrested Emon Brown and Joc Wilson, who they believed had executed Smith as part of a turf war between rival gangs in the Sunnydale Projects.

While investigators located an eyewitness who was willing to testify, her usefulness was questionable. For one thing, she said Wilson wasn't one of the shooters. To help make their case, the cops turned to physical evidence, in the form of DNA swabs gathered from the handle-grips of the bicycles supposedly used by Smith's killers, which were found at the crime scene.

On Dec. 14, 2007, Boland reported that she had a match. Two of them, in fact: In a write-up of her lab examination, she declared that both Wilson's and Brown's DNA had been found on the bikes' grips, along with the DNA of an unidentified third man.

On Jan. 31, 2008, she appeared before a grand jury, and under questioning from Assistant District Attorney Diana Garcia, offered sworn testimony echoing the findings of her report. The grand jury went on to deliver indictments of Wilson and Brown.

But there was a weakness in Boland's report and testimony, and it was one that would not be obvious to any but the expert eye. Everything she said was technically true — both Brown and Wilson matched with small traces of DNA on the grips. But a significant result of the analysis was not made clear: The third, unknown person had left much more of his DNA on the bikes, making him what forensic scientists call the "major contributor" of DNA on the crime scene evidence.

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