Federal regulators are investigating how parts made with titanium that was sold with falsified quality documentation wound up in Boeing and Airbus passenger jets that were built in recent years.

Boeing and Airbus said Friday that planes containing the parts are safe to fly, but Boeing said it was removing affected parts from planes that haven't been delivered yet to airline customers.

It will be up to regulators including the Federal Aviation Administration to decide whether any work needs to be done to planes that are already carrying passengers.

The FAA said it is "investigating the scope and impact of the issue." The agency said Boeing reported the problem covering material from a distributor "who may have falsified or provided incorrect records." The FAA did not name the distributor.

Spirit AeroSystems, which makes fuselages for Boeing planes and wings for Airbus jets, reported the falsified documents.

Boeing and Airbus declined to say how many planes were flying with parts made from the undocumented titanium.

Titanium alloys have been used for decades in aircraft production because of their light weight, strength and resistance to corrosion and high temperatures. They are used in airframes, landing gear and other parts.

Boeing said tests indicate that the parts were made from the correct titanium alloy, which raised questions about why the documentation was falsified. The company, based in Arlington, Virginia, said it buys most of the titanium it uses directly from other sources, and that supply is not affected by the documentation issue.

Boeing said it was removing affected parts on planes before delivering them to airlines. "Our analysis shows the in-service fleet can continue to fly safely," the company said. It did not say which of its aircraft models were affected.

Airbus said the parts wound up on one of its models, the A220, a relatively small airliner that is used on shorter routes.

"Numerous tests have been performed on parts coming from the same source of supply," said Airbus, which has its main offices and assembly plant in France. "They show that the A220's airworthiness remains intact."

The New York Times first reported the FAA investigation.