Washington ? The FBI has hit a major hang-up in its wiretapping surveillance program: failing to pay its phone bills on time.

Facing tens of thousands of dollars in unpaid bills, telephone companies have cut off FBI wiretaps used to eavesdrop on suspected criminals, a Justice Department audit released Thursday shows. In one office alone, unpaid costs for wiretaps from one phone company totaled $66,000.

And 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.

The audit blamed the lost connections on the FBI’s lax oversight of money used in undercover investigations, which it also said allowed one employee to steal $25,000.

Assistant FBI Director John Miller said wiretaps were dropped only a few times because of the backed-up billing, which he said didn’t significantly set back the investigations under way. He said the FBI “will not tolerate financial mismanagement, or worse,” and is working to fix the problems.