Thursday, January 16, 2014

Tres Mal Errors with DNA Evidence

A story in the Denver Post (Gurman 2014) begins with the disturbing news that
A malfunction in a DNA processing machine led to the scrambling of samples from 11 Denver police burglary cases, officials acknowledged Friday. It took more than two years for the department to discover the errors. As a result of the mix-up, prosecutors are dismissing burglary cases against four people, three of whom had already pleaded guilty.
What happened?

In 2011, "[a] machine 'froze' while running a tray of 19 DNA samples." An analyst "replaced [the samples] in the wrong order" after asking the manufacturer of the robot how to proceed. More than two years later, "the machine froze for a second time." An analyst called again and "became concerned because the directions seemed different the second time. Further review over the next month revealed" the 2011 error. Ibid.

What of It?

The police department chief of staff observed that "[n]one of the DNA was compromised; it was merely associated with the wrong case when we were done." Ibid. In other words, the crime-scene DNA profiles were mislabeled. Such errors could have helped criminals avoid detection. For instance, a burglar in case A falsely associated with case B might have had a strong alibi defense for case B. Alternatively, such labeling errors could have caused individuals to be convicted of the wrong crime -- perhaps a man guilty of a burglary could have been found guilty of an murder (or vice versa).

In this incident, however, a police spokeswoman said that "[a]ll four people had confessed to at least one burglary, but the DNA error meant they were charged with the wrong ones." Ibid. Prosecutors dismissed the charges against the four, and they will not be tried for the other burglaries because the statute of limitations has expired.

Another "tray mal" case

Misuse of automated machinery for DNA analysis also produced an error--this one involving an entirely innocent man--in "what is described as the most advanced automated DNA testing system in the UK at LGC forensics labs in Teddington." Israel 2012. The machinery extracts DNA from wells in a plastic tray. Police arrested Andrew Scott, 20, after a street fight and sent a saliva to LGC for profiling. (Doyle 2012). Instead of throwing away the tray after the run with Scott's saliva sample, however, a worker reused it in an unrelated rape case. The tray contained enough of Scott's left-over DNA to show his DNA profile in the later rape sample. The laboratory should have been aware of a problem, for "[t]he batch containing the rape sample showed DNA present in the negative control (a blank sample put through to test for contamination)." (Rennison 2012).

As a result of the error, Scott was charged with "a violent attack on a woman in Manchester – carried out when he was hundreds of miles away in Plymouth." (Doyle 2012). After he spent months in prison, the charge was dismissed. "Phone records showed he was 300 miles away on the south coast when the rape took place." Scott described the experience as a "living nightmare": "They kept me in a segregation wing which was full of rapists and paedophiles. I suffered lots of verbal abuse and other inmates spitting at us and shouting 'paedos.'" Ibid.

  •  Thanks to Bill Thompson for alerting me to the Denver case.
Copyr. (c) DH Kaye 2014

1 comment:

  1. In the Scott case, the tray was rotated 180 degrees when it was mistakenly reused on the next shift. This rotation may have helped people to diagnose the problem IIUC, and someone else's, not Scott's, DNA was found in the control. It is disconcerting that positive results in the negative controls are ignored (Andrew Scott), denied (Gary Letterman), or disregarded (Stephen Avery). As William Thompson has helped to publicize, they are sometimes also the subject of active misconduct on the part of an unscrupulous technician.