Exclusive: SFPD Concealed DNA Sample Switch at Crime Lab

DNA testing.jpg
Records of bungle in 2008 case were destroyed, investigation finds

A forensics analyst's error that resulted in a mix-up of DNA samples, reportedly in a homicide case, has been concealed by officials at the San Francisco Police Department crime lab for close to two years, SF Weekly has learned.

The sample switch, which took place towards the end of 2008, was first alleged in two letters written by an anonymous whistle-blower who worked within the lab at the time. The employee alerted both the San Francisco Public Defender's Office and the American Society of Crime Lab Directors (ASCLD) accreditation board.

When informed of the accusation by ASCLD in August 2009, SFPD crime-lab officials asserted that the complaint was unfounded, stating in a letter to ASCLD that "no instances of corrective action" were on record during the period in question for Tahnee Nelson, the DNA analyst alleged to have made the mistake.

But an interim inspection performed by ASCLD investigators this fall confirmed that "the sample mix-up did occur" and that Nelson's supervisor, then-DNA unit leader Matt Gabriel, was notified but told her to proceed with testing anyway.

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The whistle-blower letter to ASCLD. Click to enlarge.
And in what experts call a serious breach of forensics protocol, the mistake was not properly documented or shared with criminal defendants and defense attorneys.

A report on the inspection, which has not been previously disclosed, was provided by the police department in response to inquiries about the alleged sample switch last week from SF Weekly. (You can read the report here.)

The revelation is another serious blow to the credibility of the SFPD crime lab, which was battered earlier this year by reports that narcotics analyst Deborah Madden had stolen drugs for personal use. As a result, the narcotics division of the lab was shut down in March, and drug testing has been outsourced.

While public attention has focused on Madden and problems with drug testing at the crime lab, the flawed handling of evidence in the DNA-testing unit is even more troubling, some say. The DNA lab tests evidence in much more serious cases than those handled by narcotics technicians, many of them rapes and murders.

"Madden was an unfortunate character in this drama... she was really a red herring, compared to this," said defense lawyer Tony Tamburello, who has complained in the past about the quality of DNA analysis at the crime lab. "The reality is that they're not operating within the realm of sound constitutional and scientific principles. There's something drastically amiss with the crime lab."

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The response to the whistle-blower letter. Click to enlarge.
It is still unclear exactly how the sample switch took place or which cases may have been affected. Police officials say the mistake was not serious and was promptly corrected, and did not result in any aberration in the final results of the lab's tests.

Yet forensics experts say such confidence is misplaced, and that the crime lab's insistence on denying the switch took place when confronted with a whistleblower's allegations casts a shadow over all work done at the facility.

"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," said Marc Taylor, president of Technical Associates, Inc., a private crime lab based in Ventura. "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."

Public Defender Calls for Criminal Investigation


ASCLD's findings have angered some San Francisco criminal-defense attorneys, who say the District Attorney's office has not provided the document to them despite its obligation to do so. In May, following the Madden scandal, San Francisco Superior Court Judge Anne-Christine Massullo ordered District Attorney Kamala Harris to develop a policy for turning over exculpatory information about police employees and forensic work to defendants.
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