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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 — crucial 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, supervisors in the medical examiner's office thought they had gotten to the bottom of the technician's errors, which has been under internal investigation since March 2011, only to find that the trail went further, leading to more cases that had to be reviewed.

"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 widely. The office is now advocating for the acceptance of so-called low copy number DNA — sometimes transmitted only by a touch — as a way to link suspects to a crime.

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, investigators try to recover DNA evidence from saliva, semen or blood that may have been left by the assailant. The task of recovering such evidence — known as a rape kit — falls to the medical examiner's office, which currently employs 48 technicians who conduct preliminary tests on the kits.

The technician, who resigned in November 2011, had two responsibilities when processing rape kits: She had to snip cuttings from swabs taken from victims' body at the hospital, 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 the presence of 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, laboratory officials said.

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.

But in the course of reviewing the technician's work, supervisors quickly discovered another problem. Sixteen pieces of evidence, generally swabs contained in sealed 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."