Boeing evacuated some 500 employees from a plant in Pennsylvania on Tuesday after a reported bomb threat on the 11th anniversary of the September 11 attacks.
"Early this morning around about 7:30 am (1130 GMT), Boeing received notification of a threat at one of its facilities at Ridley Park," Boeing spokesman Damien Mills said.
"In the interest of safety, we evacuated employees, notified the local authorities and contacted Delaware County Police," the spokesman said.
Neither Mills nor the police would comment on US media reports that a note was left in a bathroom at the Ridley Park facility claiming explosives had been planted in one of the hangars.
The on-site investigation did not turn up any cause for concern, Mills said later.
"Because there is an ongoing investigation by the FBI we don't have any details to share at this time about the nature of the threat," he said.
After the threat was deemed to be a false alarm, the second shift at the Ridley Park plant, which manufactures the H-47 Chinook helicopter, was able to begin as normal from 2:30 pm (1830 GMT).