The problem illustrates the complex global supply chain used in producing modern jetliners, and the story of what appears to have gone wrong involves companies in China, Italy, Turkey and the United States.

The issue appears to date to 2019 when a Turkish material supplier, Turkish Aerospace Industries, purchased a batch of titanium from a supplier in China, according to the people familiar with the issue. The Turkish company then sold that titanium to several companies that make aircraft parts, and those parts made their way to Spirit, which used them in Boeing and Airbus planes.

In December 2023, an Italian company that bought the titanium from Turkish Aerospace Industries noticed that the material looked different from what the company typically received. The company, Titanium International Group, also found that the certificates that came with the titanium seemed inauthentic.

Turkish Aerospace Industries did not respond to a request for a comment.

Spirit began investigating the matter, and the company notified Boeing and Airbus in January that it could not verify the source of the titanium used to make certain parts. Titanium International Group told Spirit that when it bought the material in 2019, it had no clue that the paperwork had been forged, according to Spirit officials.

Francesca Conti, a general manager for Titanium International Group, said that the episode was under investigation and that she could not provide additional details. “We are cooperating with relevant authorities to address any issue eventually identified,” she said in an email.

The documents in question are known as certificates of conformity. They serve somewhat as a birth certificate for the titanium, detailing its quality, how it was made and where it came from, Spirit officials said.

People familiar with the situation said it appeared that an employee at the Chinese company that sold the titanium had forged the details on the certificates, writing that the material came from another Chinese company, Baoji Titanium Industry, a firm that often supplies verified titanium. Baoji Titanium later confirmed that it had not supplied the titanium. The origin of the titanium remains unclear.

“Baoji Titanium doesn’t know about the company and has no business dealing with this company,” the firm said in a statement to The New York Times.

Without knowing where the material came from or how it was handled, it is impossible to verify the airworthiness of the parts, said Gregg Brown, the senior vice president for global quality at Spirit.

“Our quality management process relies on the traceability of the raw materials all the way from the mills,” Mr. Brown said. “There has been a loss of traceability in that process and a documentation challenge.”