The San Francisco police crime-lab technician and a supervisor implicated in alleged misconduct that could jeopardize hundreds of criminal cases failed a DNA proficiency exam last year and were barred from processing evidence, documents show.

Police Chief Greg Suhr said Monday that authorities were looking at 1,400 criminal cases that were prosecuted in part based on DNA work done by the technician — identified in court records as Mignon Dunbar — and her supervisor, Cherisse Boland. Both are civilian police employees.

It’s the latest embarrassment for the crime lab, where the 2010 theft of cocaine evidence by a drug-tester ultimately led to the dismissal of 1,700 criminal cases on which she had worked.

“We’re going to do a complete audit of any work these criminalists touched,” Suhr said of the DNA tests. “If we do discover there’s any problem, all those cases will be reworked until they’re to the appropriate standard.”

Suhr said the probe will last about six weeks. In addition, District Attorney George Gascón said he has formed a task force that will look into several recent problems in the police and sheriff’s departments, including the DNA lab issues.

The lab’s problems came to light during the trial in December of a child molestation defendant. Dunbar, confronted with a low-quality sample of DNA from the assailant, allegedly made assumptions about missing data and — with Boland’s agreement — submitted two ostensibly complete genetic profiles to be compared with those of known offenders listed in a state database.

The prosecutors argued that the results pointed to the defendant, Marco Hernandez. Defense attorneys learned late in the trial, however, that one of the DNA profiles Dunbar submitted didn’t match Hernandez’s.

Hernandez had confessed to some of the case against him, however, and the jury convicted him.

Technician, boss on leave

The DNA investigation comes as a national crime-lab accrediting agency is conducting its regular five-year audit of the San Francisco lab. Without accreditation, any DNA findings the lab made in murder, sexual assaults and other criminal cases would be all but useless to prosecutors in court.

Officials with the agency, the American Society of Crime Laboratory Directors’ Lab Accreditation Board, did not respond to requests for comment.

Dunbar, 31, and Boland, 40, were reassigned from the DNA unit in September, three months after both failed a state proficiency test in DNA analysis, according to documents obtained from the public defender’s office. By then, however, both had already gone on personal leave. They declined to comment Monday.

Boland’s boss, DNA technical leader Eleanor Salmon, wrote in a memo to the head of the crime lab in September that Dunbar had failed to properly analyze a sample on the proficiency test and had come back with “inconclusive” findings. Boland approved her analysis, Salmon said.

Dunbar and Boland were the only test takers in the state to get the answer wrong, Salmon told crime lab manager John Sanchez. She wrote that their test performance was “not acceptable according to current DNA unit procedures.”

Both Dunbar and Boland, however, insisted “unequivocally that they felt their performance ... was adequate and in compliance with current unit procedures,” Salmon said. She concluded that both shared a “fundamental lack of application and/or understanding of the unit procedures in this instance.”

Dozen cases checked

Suhr said police notified prosecutors in August that Dunbar and Boland had failed the proficiency test, which prompted a review of 12 criminal cases they had worked on. “We’ve looked at those, (and) there’s no problem with them,” he said.

Public Defender Jeff Adachi, whose office defended Hernandez, said the crime lab’s actions in the case were “outrageous.”

“It is such a clear violation of basic lab protocols and a fundamental lack of understanding of how to do these tests,” Adachi said.

Questions have long dogged the lab’s DNA unit, although it avoided being tainted by the 2010 cocaine-theft scandal that resulted in police handing over all drug analyses to outside labs.

Boland was the target in 2010 of a blistering internal memo sent by a prosecutor under then-District Attorney Kamala Harris. The prosecutor, Rockne Harmon, said Boland had done an incomplete DNA analysis in the 2007 killing of a reputed gang leader in Visitacion Valley by two men on bicycles.

Two suspects were arrested, but Boland’s analysis of their bicycles found DNA from an apparent third person. Boland didn’t enter the profile into a state and federal database of convicted criminals because the material could have belonged to an innocent person, she explained when the defendants’ attorneys raised questions.

The defendants were acquitted.

Harmon sent his memo to police officials, but authorities never revealed what, if anything, they did as a result. Suhr said he did not know the outcome of the investigation, which began under his predecessor as chief, Gascón.

“She remained in that role,” Suhr said. “I would imagine that the former chief did his due diligence, and I inherited her as a supervisor. Until this happened, I had no information that her work was below grade.”

'Disturbing’ conduct

Two years later, an FBI audit of 2010 cases found that out of 100 samples of evidence the lab had submitted to the state and federal offender tracking database, seven were not up to standard and should be removed.

Deputy Public Defender Chris Schenone, who handled the Hernandez defense, said the crime lab’s conduct is “disturbing.”

“They had gotten into trouble back in 2010 and 2011,” she said, “and they still weren’t following the rules.”

Jaxon Van Derbeken is a San Francisco Chronicle staff writer. E-mail: jvanderbeken@sfchronicle.com Twitter: @jvanderbeken