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 round about 7:30 am (1130 GMT) Boeing received notification of a threat at one of its facilities at Ridley Park," Boeing spokesman Damian Mills told AFP.
"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 Boeing spokesman said the first shift had now been suspended on the south side of the plant but that they were planning to run the second shift as normal, from around 2:30 pm (1830 GMT).
"I am not able to confirm any more details about the nature of the threat," Mills said.
The Ridley Park plant manufactures the H-47 Chinook helicopter.
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