Police release new video in manhunt for Brown University attacker

Investigators searching for the killer who opened fire in a Brown University lecture hall have released new footage of a person they believe may be linked to the attack.

Four days after the gunman killed two people and injured nine others officials have still not identified a suspect, leaving the community on edge.

On Tuesday, police released "enhanced" footage of a person dressed in dark colours walking near the university two hours before the attack took place that was recorded by CCTV.

Officials are appealing for patience as the investigation proceeds, and have asked local residents to check security cameras to see if they have any footage of the suspicious "person of interest".

Footage of the person that appeared to be stitched together from home security cameras was shown to the media during a news briefing on Tuesday.

In it the man walks around the campus, with a black mask covering his mouth and nose and a black hat on his head.

Providence Police Chief Oscar Perez said the video shows the person "casing" the area before the crime.

"That's something criminals do," he said.

He also asked the public to check for anything familiar in the footage - including the way the individual walked and any other important features - and for residents in the area where the videos were taken to check if they have any of their own footage.

In one image released by investigators, the man had a black bag slung across his chest, but it did not appear in other footage.

Rhode Island Attorney General Peter Neronha said that the investigation is "going well", and told the community that detectives were working to uncover evidence.

"Rhode Islanders should take confidence in their work," he said. "They are veterans in what they do... We just need a little bit of patience."

Investigators also said the attacker was not filmed by any CCTV cameras inside the building where the attack occurred, a lecture hall where students were reviewing for an exam.

Officials so far have not commented on any suspected motive, or whether any of those killed and injured were deliberately targeted.

Meanwhile, police have dramatically increased their visible presence at the Ivy League campus, and in the surrounding community.

Officials say there is "no credible, actionable, specific threat" that has occurred there since Saturday.

Providence Mayor Brett Smiley said the "stepped-up law enforcement efforts" throughout the city are a precaution, and "just about making people feel safe".