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Police were investigating a possible hoax after Yale University was locked down for several hours Monday as police and SWAT teams found no trace of a reported gunman on the New Haven campus.

University officials instructed students and staff to "shelter in place" after an anonymous 911 caller said just before 10 a.m. that his roommate wanted to shoot up the school and was heading to the main campus with a rifle, according to police.

By 4:40 p.m., the school had lifted the shelter-in-place and lockdown orders for all parts of campus.

Most students have already left campus for the Thanksgiving holiday.

The Ivy League university issued its initial lockdown order at 10:17 a.m. ET.

Police say the tip was called in at 9:48 a.m. from a phone booth near campus. The caller said his roommate had a long gun, according to police spokesman officer David Hartman. The caller, who did not identify himself as a Yale student, stayed on the phone only a few seconds.

"All he really said was that his roommate was on his way to the university, to Yale University, to shoot people," he said.

Hartman said "several" witnesses later told police they saw someone with a rifle or shotgun on the loose.

But the call may have been a hoax, New Haven Police Chief Dean Esserman said at an afternoon press conference.

The caller "sounded like a confused gentleman. I wouldn't describe his words as clear and concise," he said.

Yale University, using Twitter, reported a gunman on or near its Old Campus on Nov. 25, 2013.(Photo: Twitter.com/Yale)

Police were interviewing several people who were not sure "if they saw a police officer with a gun or a citizen with a gun," Esserman added.

Investigators were also trying to track down the caller.

SWAT teams were dispatched in and around the campus and quickly blocked off several streets near the Old Campus, in the heart of New Haven. Several police departments, the FBI and other federal agencies were involved in the search.

The university's initial announcement said: "Confirmed report of a person with gun on/near Old Campus. Shelter in place.This is NOT a test."

"Out of an abundance of caution, Police will be doing a room to room search starting with the residential college areas," the university announced on its website. "When they knock on your door, a Yale police officer will slip their Yale ID under the door. Please cooperate. In some cases, police may use keys, but they will identify themselves. Shelter in Place continues."

The announcement was also made through the loudspeakers on campus, the New Haven Register reported.

Contributing: Associated Press

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