WASHINGTON (Reuters) -Boeing ( BA ) is asking suppliers to disclose records on Chinese titanium since 2014, according to a letter seen by Reuters, as the U.S. planemaker widens checks for false paperwork used to authenticate the metal used in commercial jets.

Regulators said in June they were investigating whether false or incorrect documents were used to identify the authenticity of titanium used for parts in some Boeing ( BA ) and Airbus jets.

Airbus said it is collaborating with authorities and investigating the lack of proper traceability affecting a small number of titanium parts from suppliers to programs like the A220, A320 and A350.

Reports of forged documentation initially raised concerns about the structural integrity of some aircraft, but planemakers and suppliers say the correct titanium alloy was used and their products are safe. Paper trails are critical in aviation, where regulators insist on clear documentation for even minor production changes to assure planes are safe.

"In the interest of full compliance, we are now broadening the scope of our request," Boeing ( BA ) wrote in the letter sent in mid-July to suppliers that asks for details by Aug. 9.

It's not clear why Boeing ( BA ) is asking suppliers to provide records for Chinese titanium purchases dating back a decade.

While the impact of the industry-wide issue is extremely limited for Boeing ( BA ), the company said it is "continuing to work with our suppliers to ensure that every titanium part is properly documented."

Aerospace-grade titanium's strength and light weight make it ideal for components that take the heaviest punishment, like engine parts and landing gear for big jets. Titanium supply has been tight due to demand for planes and as Western nations seek alternatives to metal from Russia and China.

Boeing ( BA ) has been under scrutiny all year following the Jan. 5 mid-air blowout of a door panel on a new 737 MAX 9. The required documents detailing the removal of that key part for repairs have not been located and Boeing ( BA ) believes they were never created.

Last year, jet engine manufacturer CFM International disclosed that thousands of its engine components might have been sold with falsified documentation by a British distributor.

Joe Buccino, spokesperson for Boeing ( BA ) supplier Spirit AeroSystems ( SPR ), said the company will comply with the planemaker's letter, which Boeing ( BA ) confirmed.

"Documentation compliance is critical in the aviation industry," Buccino said.

The Federal Aviation Administration (FAA) said the investigation is ongoing. The European Union Aviation Safety Agency (EASA) was not immediately available for comment.

Boeing ( BA ) asked suppliers in February to disclose whether they had procured the metal through distributor Titanium International Group (TIG) since January 2019.

In June, the New York Times reported that TIG noticed that the material looked different from previous supplies and determined that paperwork accompanying the titanium seemed inauthentic.

The planemaker has since asked its supply chain to confirm their paperwork is legitimate after the company reviewed certificates of conformance from China not recognized as authentic by the original Chinese manufacturer.