Delta Air Lines Inc. has discovered unapproved components in “a small number” of its jet aircraft engines, becoming the latest carrier and fourth major US airline to disclose the use of fake parts.

The suspect components — which Delta declined to identify — were found on an unspecified number of its engines, a company spokesman said Monday. Those engines account for less than 1% of the more than 2,100 power plants on its mainline fleet, the spokesman said.

American Airlines Group Inc., United Airlines Holdings Inc. and Southwest Airlines Co. previously disclosed finding parts from AOG Technics Ltd. on aircraft engines. That London-based company is at the center of probes by regulators into spare engine parts it sold for a longstanding type of jet engine made by CFM International Inc., a joint venture of General Electric Co. and Safran SA.

Delta would not say if the engines with the fraudulently documented parts had been on planes while they were in service. The problematic pieces, which were certified by AOG, were detected during engine work by an unnamed third party, the spokesman said.

As many as 21 of Delta’s engines could have been affected, according to Bloomberg calculations based on data provided by the airline, which would represent the most among US carriers that have disclosed a number. As many as 96 total engines were impacted by parts supplied by AOG, GE has said.

“Delta has been informed by one of our engine service providers that a small number of engines they overhauled for us contain certain parts that do not meet documentation requirements,” the company said in a statement. “Working with the overhaul provider, we are in the process of replacing those parts and remain in compliance with all FAA guidelines.”

Read more: Safran in Dark About Magnitude of Fake Jet-Engine Parts Case

None of Delta’s aircraft are currently flying with unapproved parts, and the discovery hasn’t affected flight operations, the Delta spokesman said.

Airlines, maintenance providers and regulators across the globe have been scouring records to hunt down AOG-supplied parts with forged airworthiness documents after European authorities in August determined the parts broker had supplied suspect components. Virgin Australia Airlines Pty also found engines with suspect parts.

AOG last month was ordered by a London judge to hand over records to help identify additional suspect parts after GE and Safran filed a lawsuit seeking documents relating to “every single sale of products.”

--With assistance from Ryan Beene.

(Updates with details of worldwide search in eighth paragraph)