CHICAGO (AP) — Debris has been found in the fuel tanks of 70% of grounded Boeing 737 Max jets that have been inspected by the company, Boeing confirmed on Saturday.

Inspectors found the debris in 35 out of about 50 jets that were inspected. They are among 400 built in the past year that Boeing hasn’t been able to deliver to airline customers.

Boeing temporarily halted production last month because the planes were grounded after crashes in Indonesia and Ethiopia that killed 346 people.

Although debris hasn’t been linked to those crashes, metal shavings, tools and other objects left in planes during assembly can raise the risk of electrical short-circuiting and fires. On Tuesday the company had said debris was found in “several” planes but it did not give a precise number.

The debris was discovered during maintenance on parked planes, and Boeing said it immediately made corrections in its production system to prevent a recurrence. Those steps include more inspections before fuel tanks are sealed.

“This is unacceptable and won’t be tolerated on any Boeing aircraft when it’s delivered to the customer,” the company said in a statement Saturday.

Boeing previously said the issue does not change the company’s belief that the Federal Aviation Administration will certify the plane to fly again this summer.

A Boeing spokesman cautioned against applying the 70% to all 400 jets, saying there’s no way to know how many have the same problem until they’re all inspected.

An FAA spokesman said the agency knows that Boeing is inspecting undelivered Max planes and said the agency has increased surveillance.

The number of planes with debris was reported Friday night by The Wall Street Journal.

Max jets were grounded around the world last March. Boeing is testing updated flight control software that will replace a system that has been implicated as a cause of thecrashes. The system activated before the crashes based on faulty signals from sensors outside the planes. It pushed the noses of the aircraft down, triggering spirals that pilots were unable to stop.

While investigators examining the Max accidents have not pointed to production problems at the assembly plant near Seattle, Boeing has faced concerns about debris left in other finished planes including the 787 Dreamliner, which is built in South Carolina.