The university said in a statement, "After a review of our internal records, we can confirm that Dayvon was not treated for mental illness at any of our health care facilities, nor were university officials aware of his condition."

Green attended Morgan from 2007 until he graduated with an engineering degree in May 2012, said a university spokesman, who could not immediately answer whether Green had received mental health services at that school.

Green's only prior brush with the law as an adult in Maryland was a citation from Prince George's County Police in October for allegedly drinking in a public place; the charge was dropped Feb. 5.

This was the second high-profile case on the campus in the past year. Last March, an honors student was accused of contemplating a campus shooting rampage. Alexander G. Song 2nd pleaded guilty to charges triggered by online posts that police say included the words: "hopefully I kill enough people to make it to national news." He was offered a deal without a prison sentence because he had no weapons and there was no evidence that he planned to carry out his threat.

The home where the shooting occurred Tuesday is in a residential area where many students live side-by-side with local families.

There were two other roommates who were not home at the time of the shooting, and a sixth had recently moved out, according to Parker.

Tuesday morning, a burnt pile of charcoal and a half-burnt, upside-down white plastic table could be seen in the home's backyard.

Paul Rowe, a 22-year-old College Park senior who lives across the street, said, "I heard about 10 shots go off and then we were debating whether it was firecrackers or gunshots. We figured if it was gunshots, we should stay inside."

Approximately five minutes after hearing the sounds, Rowe said, it had quieted down and he and his roommates debated calling 911, but when they looked outside, several police cars were already there.

Of his neighbors, the Westchester, N.Y., native said, "They were always out on their porch and I always wanted to go over and say, 'What's up?' But I never did. I'm glad I didn't."

Campus crime experts said that students often drop hints when they are contemplating violence, but said that such signs can be easy to miss — or tempting to ignore.

Jeff Pollard, a George Mason University psychology professor who serves as a consultant to a campus violence prevention group, said research shows that as many as three-quarters of people give hints before committing such acts.

"As we begin to peel back the shooter's history, chances are, someone, somehow was aware that there were changes in this shooter's life," said Pollard, who stressed it was too early to comment on the particulars of this incident. "Somewhere along the line he will have likely made some kind of a threat and it would be much more recognizable in hindsight."

Pollard, who serves as a consultant for the 32 National Campus Safety Index, a group formed by relatives of the victims from the 2007 Virginia Tech mass shooting, said that people often don't take such threats seriously or ignore warning signs because they don't want to get involved.

Threats — especially those involving weapons, substance abuse and drastic changes in behavior — can serve as warning signals of a violent outburst, said Gary J. Margolis, the managing partner of Vermont-based campus safety consulting group Margolis, Healy & Associates.

In the aftermath of such events, campus authorities generally comb through students' records looking for past erratic or violent acts, said Margolis, who also declined to discuss the specifics of the College Park shootings. Threat assessment teams will also speak with the suspect's professors and review court records, he said.

University police sent an email and text alert to students shortly after becoming aware of the incident; it included a description of a vehicle police were searching for. That vehicle information, from an initial police radio call, turned out to be false, police said.

In a statement Tuesday morning, Loh said, "The University of Maryland community awoke this morning to heartbreak. We are all shocked and saddened by this morning's tragic events. We extend our deepest sympathies and prayers to the families and friends of the victims."

Loh asked the campus community to "come together during this time of grief," and said the campus counseling center and places of worship will be open.

"Ours is a university of great resolve," he said. "Together, we will emerge from our collective sadness."