BAGHDAD, Dec. 3 -- The United States has begun building cases against thousands of detainees to keep them jailed after Iraq assumes legal authority over them, officials said.

Under the terms of the recently approved U. S.-Iraqi security agreement, detainees can be held only if they are charged under Iraqi law, USA Today reported Wednesday.

"We know who are the most dangerous to the security of Iraq," said Brig. Gen. David Quantock, commander of detainee operations in Iraq. "We're going to work through those files very diligently to make sure they stay behind bars."

Besides working up cases against at least 5,000 prisoners, the U. S. military also stepped up releasing detainees no longer considered a threat, USA Today reported. Military officials said they have released at least 17,500 people so far in 2008.

Building cases against prisoners considered a threat is slow because many were detained during military operations where "the focus was not evidence collection," Quantock told the newspaper.

Most files have statements and other evidence to convict, said military spokesman Marine Maj. Neal Fisher, but Iraqi judges may release some prisoners for lack of evidence.

Under the Status of Forces Agreement, U. S. forces will leave Iraq by the end of 2011. The agreement was passed last week by Iraq's parliament. If ratified by Iraq's three-member presidential council, it would go into effect Jan. 1.(UPI)