Tucson, Ariz. (AP) - Police were searching a building floor-by-floor Friday night on the University of Arizona campus after receiving a report of a man with a gun.

Authorities and school officials said there was no immediate sign of a gunman and no report of any shots fired.

Campus police received a 911 call at 4:52 p.m. about a man with a rifle at the Administration Building, and numerous officers responded, said Sgt. Juan Alvarez.

A text alert was sent to students and staff about the situation at 5:07 p.m., instructing them to stay away from the area.

The administration building and some surrounding buildings near the campus' central mall were evacuated.

Streets leading to the administration building were blocked, and dozens of police officers - some wearing helmets and armored vests - were seen on campus as a helicopter hovered overhead.

Alvarez said police were "doing a floor-by-floor search to make sure there is no threat."

"Can't say how long it will take. We are going to be thorough," he said.

Asked whether the 911 call could be a hoax, Alvarez said, "That, we don't know. We're taking it seriously."

A shooting at the University of Arizona's nursing school in October 2002 left three professors dead before the gunman killed himself.