The campus of American University, shown in an earlier photo, was locked down Wednesday after reports of an armed intruder. (Evy Mages for The Washington Post)

A lockdown that left American University faculty members, staff and students cloistered behind closed doors for hours Wednesday was lifted after a search of the campus found no evidence of a threat, the school announced.

AU shut down the campus after a report of an armed person nearby, according to a school spokesman.

On Wednesday afternoon, a message posted on the Twitter accounts for the university and its police department said there had been “reports of armed intruder near campus.” It advised those at the school in Northwest Washington to lock doors and stay away from windows.

“If outside, leave campus immediately-Police responding,” the Twitter post said.

About 1 p.m. Wednesday, a man was seen in the 3300 block of New Mexico Avenue NW, university spokesman Mark Story said in a statement. The man was reported to have an “exposed handgun,” according to the statement.

“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.

Most AU students were away from campus for the summer, but students, teachers and others who were on the school’s grounds Wednesday huddled inside offices and other rooms as law enforcement officers searched buildings. In one professor’s office, four people pushed a table to barricade themselves inside for more than two hours.

Students and employees received campus alert information through text messages and emails. People with walkie-talkies could be heard in the halls in one campus building.

An additional statement, issued by AU communications later in the day, explained that American’s campus police placed the school on lockdown about 1:30 p.m. That statement said the alert and lockdown were “in accordance with established protocols.” Authorities from multiple agencies conducted a building search during the incident, the statement said.

“When it was determined that there was no threat to members of the AU community, American University Police removed the campus from lockdown,” the statement said.

The all-clear was issued at 4:38 p.m., according to the statement.

“Our number one priority is the safety and well-being of those in our community and exercising a high degree of caution is paramount,” the statement said. “AUPD and campus administrators will conduct a comprehensive after-action review of today’s incident. AUPD will continue to work with the Metropolitan Police Department to identify the subject.”

developing story, which has been updated.