A group of high-ranking Congress members is launching an investigation into Lockheed Martin’s F-35 fighter jet program after being told U.S. military had to spend $300 million in the last five years on defective and missing parts.

The trillion-dollar F-35 program is just getting over many of the problems that plagued its early design and production and has now hit on delivery targets three years in a row and recently announced it had lowered the per-plane cost to $77.9 million.

But an investigation by the Department of Defense’s Inspector General said last year the office overseeing the spare parts program “did not receive ... Fu201135 spare parts in accordance with contract requirements and paid performance incentive fees on the sustainment contracts based on inflated and unverified Fu201135A aircraft availability hours.”

The problem with spare parts will continue to cost the military about $55 million a year, the report said.