The lockdown was partially lifted at 3:10 p.m. ET, with the exception of the Old Campus and Calhoun College sections of campus. By 4:40 p.m. the lockdown/shelter-in-place order was fully lifted.
New Haven police said a man called 911 from a payphone 1 mile from the school at about 9:30 a.m., warning that his roommate was heading to the campus to shoot people. Later, at least two witnesses reported seeing someone on or near campus with a long gun.
There were no reports of shots fired or anyone injured but police initally blocked off several streets and put the Ivy League school on lockdown in response to the anonymous phone call.
Yale officials had earlier advised students and staff to shelter in place while police performed room-to-room searches. The school in New Haven, Conn., also issued an advisory asking people who are off campus to stay away from the area.
At 11:57 a.m. ET, the school's alert page posted: "There are confirmed reports of a person with a gun on campus on the Old Campus. The shelter-in-place/lockdown continues," the university posted on its website. "Teams from Yale Police, New Haven and the State Police are on the scene and are actively searching for any gunman."
Television reports showed officers in tactical gear outside several campus buildings. A helicopter was hovering over the area. The school is on November break, with undergraduate classes to resume Dec. 2.
ABC News and The Associated Press contributed to this report.