Telephone companies have cut off FBI wiretaps used to eavesdrop on suspected criminals because of the bureau's repeated failures to pay phone bills on time.

A Justice Department audit released Thursday blamed the lost connections on the FBI's lax oversight of money used in undercover investigations. Poor supervision of the program also allowed one employee to steal $25,000, the audit said.

In at least one case, a wiretap used in a Foreign Intelligence Surveillance Act investigation "was halted due to untimely payment," the audit found. FISA wiretaps are used in the government's most sensitive and secretive criminal and intelligence investigations, and allow eavesdropping on suspected terrorists or spies.

"We also found that late payments have resulted in telecommunications carriers actually disconnecting phone lines established to deliver surveillance results to the FBI, resulting in lost evidence," according to the audit by Inspector General Glenn A. Fine.

More than half of 990 bills to pay for telecommunication surveillance in five unidentified FBI field offices were not paid on time, the report shows. In one office alone, unpaid costs for wiretaps from one phone company totaled $66,000.

Assistant FBI Director John Miller said the bureau is working to fix the problems "to ensure appropriate oversight."

"While there is widespread agreement that the current financial management system, first introduced in the 1980's is inadequate, the FBI will not tolerate financial mismanagement, or worse, and is addressing the issues identified in the audit," Miller said in a statement.

The report released Thursday was a highly edited version of Fine's 87-page audit that the FBI deemed too sensitive to be viewed publicly. It focused on what the bureau admitted was an "antiquated" system to track money sent to its 56 field offices nationwide for undercover work. Generally, the money pays for rental cars, leases and surveillance, the audit noted.

It also found that some field offices paid for expenses on undercover cases that should have been financed by FBI headquarters. Out of 130 undercover payments examined, auditors found 14 cases of at least $6,000 each where field offices dipped into their own budgets to pay for work that should have been picked up by headquarters.

The faulty bookkeeping was blamed, in large part, on an FBI employee who pleaded guilty in June 2006 to stealing $25,000 for her own use, the audit noted.

"As demonstrated by the FBI employee who stole funds intended to support undercover activities, procedural controls by themselves have not ensured proper tracking and use of confidential case funds," it concluded.

Fine's report offered 16 recommendations to improve the FBI's tracking and management of the funding system, including its telecommunication costs. The FBI has agreed to follow 11 of the suggestions but said that four "would be either unfeasible or too cost prohibitive." The recommendations were not specifically outlined in the edited version of the report.