Mayor Timothy O'Brien confirmed that one person was in custody. He was scheduled to hold a news conference with other officials later Monday to discuss the incident.
The university declared a campus emergency at about noon Monday amid reports that someone had been spotted on campus with what was believed to be a gun, said University spokesman Mark McLaughlin.
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."
The lockdown was lifted shortly after 3 p.m., and school officials said students had been safely evacuated from the residential building where police searched for the possible gunman. University spokeswoman Janice Palmer said that the students who had been inside James Hall were moved to another building for questioning.
Classes were canceled for the rest of the day at the school, which has more than 12,000 students on a 182-acre campus south of Hartford.
Mayor O'Brien said state police and departments from several surrounding towns were called in to assist with the search. The response included armored vehicles, SWAT and dog teams.
Morgan Podlisny, an 18-year-old freshman who lives in the adjacent Seth North residence hall, said students were told to stay inside. She and her friends watched the situation unfold next door.
"Our whole dorm was freaking out," she said. "At first, there were a lot of cops there rushing in with guns.
Kiara Gupton, a 21-year-old junior said she could see SWAT teams and dogs.
"I'm really just upset about this," she said, recalling last year's deadly school shooting in Newtown, about 30 miles away. "It's scary. ... To see all these SWAT teams outside is really just overwhelming."
Two New Britain schools near the campus were also locked down as a precaution, according to the superintendent's office.
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