Federal authorities have launched dozens of new criminal investigations into possible opioid and other drug theft by employees at Department of Veterans Affairs hospitals, a sign the problem isn't going away despite new prevention efforts.

Data obtained by The Associated Press show 36 cases opened by the VA inspector general's office from Oct. 1 through May 19. It brings the total number of open criminal investigations to 108 involving missing prescriptions, theft or unauthorized drug use. Most of those probes typically lead to criminal charges.

The numbers are an increase from a similar period in the previous year. The VA has pledged "zero tolerance" in drug thefts following an AP story in February about a sharp rise in reported cases of stolen or missing drugs since 2009. Doctors, nurses or pharmacy staff in the VA's network of more than 160 medical centers and 1,000 clinics are suspected of siphoning away controlled substances for their own use or street sale — sometimes to the harm of patients — or drugs simply went missing without explanation.

Drug thefts are a growing problem at private hospitals as well as the government-run VA facilities as the illegal use of opioids has increased in the United States. But separate data from the Drug Enforcement Administration obtained by the AP under the Freedom of Information Act show the rate of reported missing drugs at VA health facilities was more than double that of the private sector. DEA investigators cited in part a larger quantity of drugs kept in stock at the larger VA medical centers to treat a higher volume of patients, both outpatient and inpatient, as well as for distribution of prescriptions by mail.

In February, the VA announced efforts to combat drug thefts, including employee drug tests and added inspections. Top VA officials in Washington led by VA Secretary David Shulkin pledged a more active role, holding conference calls with health facilities to develop plans and reviewing data to flag problems. The department also said it would consider more internal audits.

Criminal investigators said it was hard to say whether new safeguards are helping.

"Prescription drug diversion is a multifaceted, egregious health care issue," said Jeffrey Hughes, the acting VA assistant inspector general for investigations. "Veterans may be denied necessary medications or their proper dosage and medical records may contain false information to hide the diversion, further putting veterans' health at risk."

In response, the VA said it was working to develop additional policies "to improve drug safety and reduce drug theft and diversion across the entire health care system."

"We have a zero-tolerance policy regarding drug theft," Poonam Alaigh, VA's acting undersecretary for health, told the AP. "We have security protocols in place and will continue to work hard to improve it."

AP's story in February included figures documenting the sharp rise in drug thefts at federal hospitals, most of them VA facilities. Subsequently released DEA data provide more specific details of the problem at the VA. Drug losses or theft increased from 237 in 2009 to 2,844 in 2015, before dipping to 2,397 last year. In only about 3 percent of those cases have doctors, nurses or pharmacy employees been disciplined, according to VA data.

At private hospitals, reported drug losses or theft also rose — from 2,023 in 2009 to 3,185 in 2015, before falling slightly to 3,154 last year. There is a bigger pool of private U.S. hospitals, at least 4,369, according to the American Hospital Association. That means the rate of drug loss or theft is lower than VA's.

The VA inspector general's office said it had opened 25 cases in the first half of the fiscal year which began Oct. 1. That is up from 21 in the same period in 2016.

The IG's office said the number of newly opened criminal probes had previously been declining since 2014.

Michael Glavin, an IT specialist at the VA, says he's heard numerous employee complaints of faulty VA technical systems that track drug inventories, leading to errors and months of delays in identifying when drugs go missing. Prescription drug shipments aren't always fully inventoried when they arrive at a VA facility, he said, making it difficult to determine if a drug was missing upon arrival or stolen later.

Congressional auditors this year found that at least four VA hospitals skipped monthly inspections of drug stocks or missed other requirements, even after warnings about lax oversight dating back to at least 2009.

"It's still the same process," said Glavin, who heads the local union at the VA medical center in Columbia, Missouri. The union's attorney, Natalie Khawam, says whistleblowers at other VA hospitals have made similar complaints.

Criminal investigators stressed the need for a continuing drug prevention effort. The VA points to inventory checks every 72 hours and "double lock and key access" to drugs. It attributes many drug loss cases to reasons other than employee theft, such as drugs lost in transit. But the DEA says some of those cases may be wrongly classified.

"Inventories are always an issue as to who's watching or checking it," said Tom Prevoznik, a DEA deputy chief of pharmaceutical investigations. "That would always be part of any investigation we do, asking 'What are the employees doing, and who's watching them?'"

The Senate is expected to vote June 6 on VA accountability legislation that would give the agency "the tools necessary to remove employees who are failing to perform at the high-quality level." A lead sponsor of the bipartisan bill, Sen. Marco Rubio, R-Fla., pointed to AP's findings as "troubling."

"The theft and misuse of prescription drugs, including opioids, by some VA employees is a good example of why we need greater accountability at the VA," he said.