The university declared a campus emergency late Monday morning, and several hours later its website continued urging students to remain inside buildings.
University spokesman Mark McLaughlin said, "Somebody was seen either with a gun or was thought to have a gun."
Jordan Governale, a 20-year-old junior from Farmington, said he walked by a man carrying a backpack and with a sword and sheath strapped to his back. The man was wearing a mask, camouflage pants, knee pads and a vest resembling body armor, Governale said.
A minute later, he said, he saw police.
"At first I thought it was a Halloween costume. But after I saw the cops I thought it was some sort of threat," he said. "It's pretty scary. It's pretty strange, unexpected."
Police took 3 people into custody, and an all-clear was issued around 3:00 p.m. Authorities described it as an isolated incident. No shots fired.
Investigators said no weapons had been recovered, but they are still looking. Charges have not been filed yet.
Despite the lifting of the lockdown, the university has cancelled classes and other events scheduled for this afternoon and tonight.
The incident began shortly after noon, when CCSU began alerting students to stay inside and lock all windows and doors because of an emergency.
CCSU's Twitter account sent out messages such as "stay indoors this is not a drill. Campus emergency. Find shelter until further notice."
School officials said police urged officials to alert everyone on campus to get inside aftert he report of a suspicious person on campus.
A road leading onto campus was closed and police officers were seen sheltering behind campus walls as agencies from several nearby towns responded. The area around a 400-student dorm was the focus of the police search, but by mid-afternoon authorities were allowing students to leave the building in small groups.
No injuries were reported.
More than 12,000 students attend the university, which has a 182-acre campus