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.