WASHINGTON (WASHINGTON POST) - A lockdown that left American University faculty members, staff and students cloistered behind closed doors on Wednesday (July 18) has been lifted after a search of the campus found no evidence of an armed person, the school announced.

AU shut down the campus after a report of an armed person nearby, a decision made "out of an abundance of caution," according to a school spokesman.

"Reports of armed intruder near campus," the university's police department wrote on Twitter. "Lock doors & stay away from windows. If outside, leave campus immediately. Police responding."

About 1pm on Wednesday, a man was seen near the campus. The man was reported to have an "exposed handgun," according to a statement by university spokesman Mark Story.

"While the initial report indicated that this person was not on the AU campus, out of an abundance of caution, the American University Police Department issued an AU Alert and directed that the campus be put on lockdown," Story said.

The university's Twitter account posted an all-clear message later on Wednesday afternoon, saying that buildings and other areas on the grounds were "deemed secure" and university operations would return to normal.

The Washington DC university had issued a tweet saying a building-by-building search was being conducted by campus and District of Columbia police.

In one professor's office, four people pushed a table to barricade themselves inside for more than two hours.