In an email later on Monday night, Peter Kyem elaborated. He said that his son was returning to the campus on Monday after spending the weekend at the University of Connecticut, where he had attended a Halloween party. His son was wearing his costume when he arrived back at James Hall, where he lives.

“It was then that some who saw him enter the hall called the police and the frenzy began,” he said.

University officials did not release further details about what had happened, saying it would update the community when relevant information was confirmed. Officials said that there were no reports of injuries, that counselors would be available for appointments on campus, and that classes would resume Tuesday. Earlier Monday, the university said no weapons had been found.

Campus police, the State Police and the Police Departments of New Britain and Newington conducted the investigation. Initially, three people described by the campus police chief as “school age” were taken into custody in a dormitory room before the lockdown order was lifted. Officials had said they were looking into whether the outfit that had caused the alarm had been a Halloween costume. The university declared a campus emergency after the 911 call, which sent New Britain police officers to the campus almost immediately. State troopers followed, and soon police helicopters were swirling overhead.

A student who said he called 911, Nicholas Federici, said in an interview that the man he had seen was wearing a black vest with pockets for ammunition and black kneepads. In one hand, Mr. Federici said, the man was holding a backpack. In his eyes was “a thousand-yard stare.”

He said the man went into James Hall, an eight-story dormitory that is home to more than 400 of the university’s nearly 12,000 students. The man dropped the backpack — “I heard a big thud,” Mr. Federici said — but had disappeared inside by the time a university security officer arrived moments later.