(CNN) -- SWAT teams were searching Yale University buildings Monday after someone anonymously told a 911 dispatcher that his roommate was coming to the southern Connecticut university to shoot people, police in New Haven said.
No gunfire and no injuries were reported as of Monday afternoon. Most of Yale's schools were out for Thanksgiving recess, but some students and staff remained, and the school sent e-mails and texts asking them to remain in locked rooms or offices.
Some people subsequently reported seeing a person with a rifle or a shotgun on or near campus, though police were trying to determine whether the sightings were of investigating police officers, New Haven police Officer David Hartman said Monday afternoon.
The call virtually shut down Yale's Old Campus, where police were focusing their search, letting no one in or out of the area, Hartman said.
The shelter-in-place order was slowing officers' room-to-room searches of dorms and other buildings, because the people inside often weren't immediately opening doors for the police, not trusting that the police had come to check, Hartman said.
Many of the doors, he said, didn't have didn't have peepholes, so officers were taking their time persuading the occupants.
\"If Yale were in full session right now, this certainly would be a tougher job for law enforcement,\" Hartman said.
Police said someone called 911 from a pay phone around 9:30 a.m. and matter-of-factly said his roommate \"was on his way to Yale university to shoot people,\" Hartman said.
Investigators didn't know who the caller was or who the supposed roommate is, he said. He wouldn't comment on whether the call might be a hoax.
\"We aren't taking anything for chance and we'll continue to investigate each report,\" he said.
It could be hours before police give an all-clear for the campus, Hartman said at a 2 p.m. news conference.
Students were allowed to stay on campus during Thanksgiving week, but most had left by Monday, school spokesman Tom Conroy said.
The university issued its first alert at 10:17 a.m. Less than an hour later, the university said there was a \"confirmed report of a person with a gun on/near Old Campus\" -- apparently referring to the witness reports that Hartman mentioned
\"Teams from Yale Police, New Haven and the State Police are on the scene and are actively searching for any gunman,\" the university said later on its website.