The F-35 fleet is safe to fly, despite an unknown number of under-strength fasteners being used to build critical areas of the jet, Pentagon acquisition and sustainment chief Ellen Lord said Jan. 31.

Lockheed Martin workers mixed up titanium and Inconel bolts during manufacture of the F-35, and the Defense Contract Management Agency told Air Force Magazine neither the company nor the Joint Program Office knew how many aircraft were affected, or how far back the problem started. It said the whole fleet of 400-plus F-35s could potentially be affected. The titanium fasteners are lighter than the Inconel parts, and also have less shear strength.

Lockheed is to present its 70-day root cause analysis of the “quality escape” to the government in February.

At a press conference to discuss cyber security rules for Pentagon contractors, Lord said she had “looked at samples of that issue”—meaning the mixed-up fasteners—and said “right now we have assessed that there is no structural compromise of the aircraft.” She said the root cause analysis continues.

“The JPO is working closely with Lockheed; we will continue to asses if there are any issues, but we have confidence in the integrity of the aircraft at this point.”

Read more: air force mag