The FBI has been slammed for wasting over $10m (£5.8m) on "questionable contractor costs" and another $7.6m on missing equipment.
In a report, the US Government Accountability Office (GAO) has criticised a number of FBI "weaknesses" in its financial dealings with contractors.
These include incorrect overtime payments, potentially inflated wages, and excessive and first-class airfare costs.
In one example covered in the report, Computer Sciences Corporation (CSC), an FBI sub-contractor, charged the bureau over $456,000 for services described only as "other direct costs."
The GAO said that, after an inquiry it was found that CSC may not have had enough information to approve the charge, but billed it to the FBI anyway.
Another contractor, CACI, hired to do training for the FBI, billed the agency $50,000 to cover the cost of custom-made highlighters and pens.
The GAO also criticised billed air travel costs.
Government employees and contractors are supposed to fly economy, unless special permission is given. But the GAO found 19 first-class tickets that cost more than $20,000, and 75 "unusually expensive" economy tickets costing more than $100,000 collectively, that could not be justified by either the FBI or contractor CSC.
The FBI is also said to have lost 1,200 pieces of equipment, including desktop and laptop PCs, printers and servers.
The GAO said there was an over-reliance on contractors to keep adequate records of equipment used. It said the FBI should adopt better record keeping for equipment and pay closer attention to contractor expenses billed.
Responding to the criticism, the FBI said it accepted the GAO’s recommendations and that it had “accounted for” 1,000 of the missing items.
The expenses highlighted by the GAO cover a period when the FBI was going through a wide-scale IT infrastructure upgrade.
The FBI completed the building of a new IT infrastructure in 2004, but it failed to deliver a planned new case management system for investigations, after a previously adopted system hit problems.
The FBI last week said the main contract for a new $425m case management system, known as Sentinel, had been awarded to Lockheed Martin.