NEW HAVEN, Conn. (AP) — Yale University was locked down for nearly six hours Monday as authorities investigated a phone call saying an armed man was heading to shoot up the school, a warning they later said was likely a hoax.

SWAT teams searching the Ivy League campus did not find a gunman after a room-by-room search and the lockdown was lifted Monday afternoon. No one was injured, police said.

"New Haven is safe. The Yale campus is safe," said New Haven Police Chief Dean Esserman.

A 911 call was received at 9:48 a.m. from a man at a pay phone about a mile from the campus who said his roommate was on the way to the university to shoot people, said Officer David Hartman, a New Haven Police spokesman.

Esserman said he was leaning toward the incident being a hoax and that a witness who reported seeing someone with a rifle likely saw a law enforcement officer.

"Though it is starting to tilt in the direction of an innocent mistake, it started with a purposeful and malicious call," Esserman said, vowing to track down and arrest the person who made the call.

Authorities do not believe the caller was a Yale student or that his roommate attended Yale, Esserman said. There was nothing specific about the threat, he said, and the call lasted only seconds.

Many students and staff members left campus for the Thanksgiving holiday following Saturday's traditional football game against Harvard.

But many were still in their dorm rooms, Hartman said, and Yale authorities sent out their first warning at 10:17 a.m., about a half hour after the 911 call.

"The Yale police made the right call," Esserman said. "They went to immediate lockdown to keep everybody safe."

Yale advised students and staff members to shelter in place. The school also issued an advisory asking people off campus to stay away from the area. The shelter advisory was lifted by late afternoon.

Police blocked off several streets near the university's Old Campus, in the heart of New Haven, where they were concentrating their search. Several local schools also were placed in lockdown. Police in tactical gear entered several campus buildings. Pedestrian traffic in the normally bustling area was sparse, with cold and windy weather keeping many people inside.

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The response included several police departments, the FBI and other federal agencies, Hartman said.

Police had difficulty gaining access to some rooms because those locked inside were not convinced they were dealing with law enforcement, he said. Most rooms don't have peepholes. Yale sent out an email telling community members that officers would be slipping a Yale ID under the door or using keys to gain access.

Undergraduate classes are set to resume Dec. 2

Yale has been the target of violence in the past. In May 2003, a bomb damaged an empty classroom and adjacent reading room at the law school.

A Yale professor, David J. Gelernter, was seriously injured in 1993, when a mail bomb mailed by Theodore Kaczynski, the man known as the Unabomber, exploded in his campus office.

Monday's search came several weeks after a scare on another Connecticut campus.

Central Connecticut State University was in lockdown for several hours Nov. 4 after reports by witnesses of a masked man carrying a gun or sword.

Police arrested a student, David Kyem, who said he had been wearing a ninja-like Halloween costume and meant no harm. He faces charges including breach of peace.

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Associated Press writer Pat Eaton-Robb in Hartford contributed to this report.