As a result, some of the agencies that pay OPM to give a green light to a new hire are turning to outside contractors.

The cost for each investigation climbed even as the number dropped, to $1.2 million in fiscal 2011 from $1.7 million three years earlier, auditors said.

“OPM presents only very broad reasons for price increases and has never provided customer agencies with detailed explanations of why it needs to increase prices in most years, but not in some others,’’ the report says.

Personnel officials attribute the increase to higher Federal Bureau of Investigation charges to check its fingerprint database, more in-depth interviews of applicants and tighter deadlines.

Auditors found other reasons too: Increased pay and benefits for OPM investigators and the agency’s IT investments, which have increased dramatically.

The GAO recommends that OPM give federal agencies more details about the costs of a background check— and identify ways to save the government money.

OPM chief John Berry agreed, but said auditors failed to take into account his agency’s “incredible improvements” in reducing a long-criticized backlog for background checks. They now take 40 days on average down from 145 days in 2005.