The New York City medical examiner’s office is undertaking an unusual review of more than 800 rape cases in which critical DNA evidence may have been mishandled or overlooked by a lab technician, resulting in incorrect reports being given to criminal investigators.
Supervisors have so far found 26 cases in which the technician failed to detect biological evidence when some actually existed, according to the medical examiner’s office. In seven of those cases, full DNA profiles were developed — in some instances, evidence that sex-crime investigators did not see for years, hampering their ability to develop cases against rape suspects.
In one of those instances, the newly discovered DNA profile matched a convicted offender’s sample, leading to an indictment a decade after the evidence was collected, according to Dr. Mechthild Prinz, the director of forensic biology at the medical examiner’s office.
In two other instances, the new DNA profiles were linked to people either already convicted or under suspicion.
The scope of the problem has yet to be determined; at several points over nearly two years, supervisors in the medical examiner’s office thought they had gotten to the bottom of the technician’s errors, only to find that the trail went further.
“This is the first time we’ve had anything like this,” said Ellen Borakove, a spokeswoman for the office of the chief medical examiner.
The office has been at the forefront of forensic technology; its work after the Sept. 11 terrorist attacks and in trying to name the unidentified dead in the city’s potter’s field has been hailed. The office, which handles evidence from about 1,500 sexual assault cases a year, is now advocating the acceptance of so-called low copy number DNA — sometimes transmitted only by a touch — as a way to link suspects to crimes.
But the continuing review of the technician’s cases underscores how DNA evidence, widely perceived as being nearly irrefutable proof of guilt or innocence, can still be subject to human error.
Each time there is an accusation of a sexual attack, DNA evidence from saliva, semen or blood that may have been left by the assailant is collected in a rape kit. The task of analyzing the evidence falls to the medical examiner’s office, which employs 48 technicians who conduct preliminary tests on the kits.
The technician had two responsibilities when processing rape kits: She had to snip cuttings from swabs taken from victims’ bodies and place them in test tubes for DNA analysis by more experienced lab workers.
She also inspected the victims’ clothing, usually underwear, for stains that might indicate DNA. Sometimes she overlooked stains, the review found. At other times, she identified stains, but then botched the chemical test used to detect semen and reported finding nothing.
The errors, Dr. Prinz said in an interview, involved reporting false negatives, not false positives. “We do know that nobody was wrongfully convicted,” she added.
The medical examiner’s office declined to publicly identify the technician, who worked there for nine years. She resigned in November 2011, after the office moved to terminate her employment, according to documents.
Her work fell under scrutiny after she enrolled in a training program to become a DNA analyst. As part of the program, she worked on a broader range of cases, including homicides, but her supervisors grew concerned as they “corrected deficiencies within cases,” according to a letter from the medical examiner’s office. The mistakes persuaded the office to look at her earlier work. The office has not yet concluded its review of 412 cases out of 843 it intends to examine, Mr. Lien said. The cases span from 2001 to 2011.
In the course of reviewing the technician’s work, supervisors discovered another problem. Sixteen pieces of evidence, generally swabs sealed in paper envelopes, were found in the wrong rape kit, commingling DNA evidence from 19 rape investigations, according to a letter from the medical examiner’s office.
“Our guess is the technician had both kits open at the same time, and when she was reassembling the case files, evidently she had misplaced the evidence items from one kit to another,” Eugene Lien, a quality assurance manager with the medical examiner’s office, told a state oversight board last year. It was not “standard policy at all,” he added, for a technician to have two cases open at once.
Asked in an interview about the risk of cross-contamination, Dr. Prinz said: “It’s extremely unlikely. I don’t think that is a risk.”
But William C. Thompson, a criminology professor at the University of California at Irvine, said that “accidental transfer in a lab of DNA from one sample to another sample is not a rare event.” He estimated that, nationwide, cross-contamination of samples is found at a rate 1 in 100 cases or higher.
Professor Thompson said he knew of three people in the United States who were wrongfully convicted as a result of contaminated DNA testing or mistaken reading of DNA tests.
In an e-mail, Erin Murphy, a law professor at New York University, said that such a basic lapse in protocol also raised the question of “what other basic rules of good forensic practice” the technician might have ignored.
“Is an analyst with such a callous disregard for the integrity of the evidence and the seriousness of his/her job one also likely to disregard other rules, like changing gloves or cleaning the workstation or other methods necessary to safeguard the evidence?” wrote Professor Murphy, who has written about DNA labs.
A lawyer for the medical examiner’s office, Mimi Mairs, said the agency had committed to “leaving no stone unturned in recalling casework she touched.”
“We have an enormous public trust to produce accurate results, and we have accordingly dedicated ourselves to do the most comprehensive review of her cases,” Ms. Mairs said.
In September, in reviewing hundreds of the technician’s cases that were recently recalled from storage, staff members worked overtime shifts.
“We call it a jamboree,” Mr. Lien told the oversight board. “If we make a mistake, obviously we have to do whatever it takes to make sure the results are reported correctly,” he said.
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