The American aircraft company Boeing has announced the discovery of damaged wiring in as many as 10 of its 737 planes in final assembly.
Security at the plant, in Washington state, has been stepped up.
Company spokeswoman Sandy Angers said seven cases of wire damage "not common in the normal course of manufacturing" had been discovered at the Renton plant.
This prompted Boeing to review other earlier incidents of unusual damage, which it now considers suspicious.
Ms Angers said the damage had been discovered during routine testing.
"Airplanes go through rigorous testing by Boeing, the Federal Aviation Administration (FAA) and the airlines before they're delivered," she said.
"This proves our quality system works."
Each aircraft showed just one incidence of damage, in different places in each plane.
The company notified the FAA earlier in the week after a tip-off from an employee on the factory floor.
The FAA passed on the findings to the FBI, which is considering whether to investigate.
Tampering with an aircraft is a federal offence in the United States.
Ms Angers said that if a company employee was found to be responsible for the damage Boeing would take "swift and immediate action".
But she said that the vast majority of Boeing workers took great pride in their work.
"If this damage proves to be intentional there are a lot of people that would be very, very hurt," she added.
The twin-engine 737 is Boeing's most popular plane. The Renton assembly plant produces around 24 of the planes a month.