A week ago, authorities were called to the campus dorm room of Alexander G. Song II, 19, who, according to the University of Maryland College Park police chief, was “shouting and acting out.”

The sophomore told officers he was “very stressed out,” the chief said, but “there was nothing that led us to believe he was a threat to us or to others.” But by Sunday, police had Song in custody, saying he posted messages on the Internet threatening “a shooting rampage on campus.”

During a news conference Monday, University Police Chief David B. Mitchell described Song as “very emotionally distraught” when he was arrested Sunday morning. “He was shaking and crying.”

How this student went from the heights of academia — graduating from high school near the top of his class and enrolling in an elite college research program — to being placed in a psychiatric ward and charged with threatening to shoot up the campus remains a mystery.

Mitchell said Song was “in good academic standing” and was accepted into an honors program in which his science study group was trying to purify methane gas to turn it into an alternative energy source.

School officials would not comment further on how he was doing on campus or what might have caused him stress. Song's parents, who live in Howard County, did not respond to interview requests.

According to campus police, Song posted comments Saturday on Web sites that said “I will be on a shooting rampage tomorrow on campus,” and “hopefully I kill enough people to make it to national news.” Another post, police said, warned: “stay away from the Mall tomorrow at 1:30.”

There was no indication that Song possessed weapons or even had access to them, Mitchell said, and he was unarmed when officers arrested him Sunday morning on campus. School officials did not put out a campus alert until 1 a.m. Monday, hours after rumors started to flourish and the story first appeared in the media.

In a statement, university President Wallace D. Loh said that detectives were “actively tracking the student’s whereabouts throughout [Sunday] morning, and a public alert might have disrupted those efforts before they were able to take him into custody.”

Loh added that police “are confident that any threat to our community was mitigated” after Song was arrested and his campus privileges suspended.

Police have charged Song in a warrant with disturbing the orderly conduct of the activities, administration or classes at the campus, a misdemeanor that could result in a $2,500 fine or six months in jail. Police said the warrant will be served after Song completes counseling at a psychiatric hospital, where he was taken for observation.

LaVonna Livingston, a sophomore, said Song lived in her building, Oakland Hall, a dorm of suites that holds as many as 700 students. “I consider this a really safe campus,” Livingston said. “Just to have the possibility of something like this happening here, it’s definitely still on my mind.”