PHILADELPHIA (Reuters) - A Boeing Co. facility near Philadelphia returned to normal operation on Tuesday after hundreds of workers were evacuated because of a threat against the company, authorities said.

Boeing spokesman Damien Mills said police found "nothing untoward" at the plant, which is located in Ridley Township, south of Philadelphia. He said the investigation was continuing.

Earlier in the day, an unspecified threat made against the company resulted in the evacuation of about 500 workers, part of the 6,200 employees who make CH-47 Chinook helicopters and the V-22 Osprey aircraft at the sprawling facility.

Mills declined to reveal the nature of the threat.

The evacuation came amid tensions surrounding the anniversary of the September 11, 2001, attacks, in which nearly 3,000 people were killed by airliners hijacked by Islamist militants.

It also coincides with heightened concerns about workplace violence, including an episode in New York last month when a man shot and killed a former co-worker near the Empire State Building and an incident in suburban New Jersey when an employee shot two co-workers before taking his own life.

(Reporting by Dave Warner; Editing by Xavier Briand)