DNA evidence has gone untested in more than 800 rape and sexual assault cases even though detectives from the Los Angeles County Sheriff’s Department and other smaller agencies have no suspects in those crimes, authorities acknowledged Tuesday.

The number is far larger than officials had anticipated and revealed a breakdown in the way the Sheriff’s Department went about testing genetic evidence until recent reforms were enacted. For months, sheriff’s officials sought to downplay concerns over a massive backlog of untested DNA evidence by suggesting that the crimes had been resolved by other means.

Compounding the problem was the revelation that in 51 of the 815 cases tallied so far, the genetic evidence has sat untested in county storage freezers for more than a decade -- so long that state laws now prohibit officials from arresting anyone even if belated genetic testing were to definitively identify a suspect.

“You’ve got a bunch of evidence sitting there that is potentially a smoking gun,” said L.A. County Supervisor Zev Yaroslavsky to a contrite Cmdr. Earl M. Shields, who oversees the department’s Technical Services division. “It could be the silver bullet to getting a suspect in a sexual assault case, and it’s just sitting there.”

Shields reported the troubling figures as part of a presentation to county supervisors on the sheriff’s ongoing attempt to work through a backlog of untested samples of semen, saliva, blood and other genetic evidence collected from victims after an alleged sexual attack. Currently 4,738 of the so-called sexual assault kits in county storage facilities remain untested -- about 20% of them from other police agencies in the county that rely on the sheriff’s crime laboratory for DNA analysis.

Sheriff’s officials have managed to gather information on only about two-thirds of the backlogged cases so far, meaning that the number of investigations without any suspects or those that have fallen out of statute is likely to rise.

Until late last year, the Sheriff’s Department had followed a policy of testing DNA evidence only when investigators in the case requested it. After coming under pressure from advocacy groups and the Board of Supervisors about the growing backlog, Sheriff Lee Baca announced in November that the department’s lab would test DNA evidence in all cases regardless of whether the analysis was requested.

At the time, the Sheriff’s Department -- as well as the Los Angeles Police Department, which came under similar scrutiny -- tried to minimize the significance of the size of the backlog. Both agencies indicated that the vast majority of untested kits were from cases in which investigators had determined the genetic evidence was not needed.

In November, Shields told supervisors that he expected an inventory of the sheriff’s backlog would uncover very few, if any, examples in which investigators had no leads on suspects and had not asked for DNA testing. “There should not be any [such] cases,” he said. “We’re hoping that that number will be zero.”

On Tuesday he conceded under questioning from Yaroslavsky that “we were hoping the number would be much smaller.” Without detailed information on each investigation, Shields was at a loss to explain why so many investigators had not asked for the evidence to be tested. He speculated they might have decided they “didn’t have a valid, prosecutable case” because the accuser recanted or other fundamental problems arose.

“It was a judgment call on the part of the investigator,” he said.

Sarah Tofte, a researcher with Human Rights Watch who has been pressing local law enforcement agencies around the country to address backlogs, questioned that logic. “Investigators may think that a victim’s account lacks some veracity,” she said, “but when someone reports to police that she’s been raped, the default should always be, ‘Let’s test this kit and see what we find.’ ”

Unexamined evidence kits hold potentially crucial information. Through a complex scientific process, DNA analysts can extract a person’s genetic code from the collected samples and compare it to those of known felons that are kept in federal and state databases. When the DNA sample collected at a crime scene or from a victim’s body matches a DNA profile of someone in the database, it can offer prosecutors nearly irrefutable proof of the person’s guilt. The evidence can also be used to confirm that someone has not falsely confessed to a crime or link someone to other unsolved cases.

LAPD Deputy Chief Charlie Beck declined to reveal how many cases for which LAPD detectives have no suspects and have not pursued DNA testing. A recent inventory of the LAPD’s untested kits has been completed, but the results are not yet final, he said.

Like the LAPD, the Sheriff’s Department has struggled to devise a financially feasible plan to eliminate its DNA backlog. Both agencies are currently increasing the number of in-house lab analysts to handle the constant influx of new cases, while also plotting out ways to outsource the backlogged cases to private labs. Each kit costs about $1,000 to process.

Shields told L.A. County supervisors that he would soon present Baca with a proposed plan and said it would take “years” to clear the backlog. In an interview, he declined to provide specifics.

Time is a major factor: More than 100 sheriff’s cases are within six months of reaching the state’s 10-year statute of limitations, Shields reported.

j oel.rubin@latimes.com