Boeing 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 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.