WASHINGTON (NewsNation) — The four largest U.S. airlines have temporarily removed some planes from service after finding engine parts with counterfeit safety certificates.

Delta, United, American and Southwest Airlines are now working with federal regulators and foreign counterparts in a global effort to find out where the fake safety certificates came from, and how it could have happened in the first place.

Southwest and Delta confirmed to NewsNation that they’ve inspected their fleets for the possibly improperly-certified engine parts. Southwest says they have not specifically identified any of those parts in their fleet. Delta did not respond to a NewsNation request for comment.

Jet engine maker CFM International said Wednesday the number of engines suspected of containing falsely documented parts from a U.K. distributor had risen to 126. The parts were, in some cases, low-pressure turbine blades, but were largely less important parts like nuts and bolts.

The number, however, still represents less than 1% of the 22,600 CFM56 engines in use globally, according to Reuters.

Without accurate paperwork, airlines cannot tell which parts are airworthy. Suspect or unapproved parts have to be removed.

CFM56 engines entered service in 1982 and power the previous generation of Boeing 737s and about half the previous generation of Airbus A320s. These are gradually being upgraded to newer models, but thousands remain in service.

The CFM56 is the most-sold jet engine in history and was developed by a transatlantic venture that turns 50 next year.

The Federal Aviation Administration (FAA) did not respond to a NewsNation request to comment.

Reuters contributed to this report.