A Pentagon project to continuously modernize the F-35 stealth jet has seen about $2 billion in cost overruns since 2019, according to a new report from the Government Accountability Office (GAO).

The report, released Thursday, said the program, designed to upgrade the jet with new capabilities was beset by an unrealistic schedule as well as a cost increase for flight test results, spiraling overhead and administrative costs as well as the cost of a new training lab and computer system upgrades.

That means the Department of Defense (DoD) and Lockheed Martin haven’t been able to consistently follow their plan for Block 4 upgrade work on the fighter.

The report estimates that Block 4 development is now projected to reach $14.4 billion.

The modernization effort was originally expected to wrap in 2026 and is now expected to wrap a year later.

But the GAO report said that schedule is based on “estimated at the start of the Block 4 effort” and that the scheduled 2027 completion date is not achievable.

Earlier this week Kyra Daley, director of strategic communications for the Pentagon’s F-35 Joint Program Office, said the DoD is months away from declaring the F-35 ready for full-rate production due to difficulty integrating the jet with the DoD’s Joint Simulation Environment.