Police officers were going door-to-door on Monday seeking footage from home surveillance cameras as investigators renewed a manhunt for the gunman who killed two students and injured seven more in a classroom at Brown University.

The search for the suspect, which included posting new video footage of the possible shooter, resumed after authorities released a man they had detained over the weekend as a "person of interest."

The news that the gunman remained at large put Providence back on edge, though officials said there were no credible threats to the community and that they would not reimpose a shelter-in-place order for the campus and the surrounding area that had been lifted earlier.

"People are very confused and nervous," said Sue Erkkinen, a real estate agent.

"We are staying indoors. We have all been glued to the TV, and it looks like the manhunt is now back on."

Providence police chief Oscar Perez told reporters at an evening press conference that law enforcement was trying to reassure residents by keeping a visibly high profile in the community.

Perez played three short video clips from home surveillance footage of the suspect, a man of stocky build dressed in a dark jacket, hat and face mask, walking through the College Hill neighbourhood near campus about two hours before the shooting.

One showed him pacing along a white picket fence, another crossing a street corner and another walking past a gated home.

Two still photos of him on a sidewalk were also shown. But the man's face was obscured by the mask he wore.

"The sooner we can identify this person, the sooner we can blow this case open," state Attorney General Peter Neronha said at the briefing.

An FBI special agent in charge from Boston, Ted Docks, said a US$50,000 reward was being offered for information leading to the identification, arrest and conviction of the suspect, who he said was presumed to be armed and dangerous.

Perez said the murder weapon was a 9mm gun from which several rounds were fired.

The gunman fled after opening fire on Saturday in a classroom in Brown's Barus & Holley engineering and physics building, where outer doors had been left unlocked while exams were taking place, according to police.

Students spent hours barricaded in classrooms or hiding beneath furniture as officers fanned out across campus searching for the attacker.

Officials said late on Sunday there had been enough evidence to justify taking into custody the unnamed person of interest, a man in his 20s.

The announcement of the detainment early on Sunday provided what turned out to be a short-lived measure of relief for students and city residents.

Neronha said hours later that investigators had determined there was "no basis to believe that he's a person of interest, so... he's being released."

Brown is one of the oldest and most prestigious universities in the United States.

The Ivy League school, which has nearly 11,000 undergraduate and graduate students, cancelled exams and classes for the rest of the year. (Reuters)