Police and the FBI have released "enhanced" video footage of the suspected Brown University shooter and called for the public to help identify him.
Two students were killed and another nine people were injured in the shooting on the Ivy League school's campus in Providence, Rhode Island, on Saturday afternoon, local time.
As the manhunt for the shooter entered its fourth day, authorities released a montage of security camera footage that showed a masked man walking around the residential streets near the university campus in the hours before the attack.
Oscar Perez, Providence's police chief, said authorities believed the footage showed the suspected gunman "casing out this area to commit the crime".
He said authorities released the footage in the hope it would help a member of the public identify him, with the FBI offering a reward of up to $US50,000 ($75,000) for information that leads to the killer's capture.
"You want to focus on the body movements — the way the person moves their arms, the body posture, the way they carry their weight," Colonel Perez told reporters.
"These are important movement patterns that may help you identify this individual."
Colonel Perez said a witness told police they believed the suspect was aged in his 30s. He said police had not yet determined the motive or whether the shooter had any connection to the university.
He also asked residents in the area to check if they had any more footage from "at least a week" before the attack.
"We also know in this profession that many criminals … case out an area weeks and days prior [to committing a crime]," he said.
'Profound sorrow'
The university's president Christina Paxson said Brown remained "deeply committed to the safety and security and wellbeing of our community".
"I have been deeply saddened by people questioning that," she told reporters.
"As time goes on, there is a natural instinct to assign responsibility for tragic events like this. Anxiety here is very natural, but the shooter is responsible."
The two students killed in the attack have been identified as Mukhammad Aziz Umurzokov, 18, from Virginia, and Ella Cook, 19, from Alabama.
"Both were brilliant and beloved as members of our campus community, but even more by their friends and families," Ms Paxson said in a statement.
"Our hearts continue to be with them in their profound sorrow."
Ms Cook was the vice-president of the school's College Republicans student group and a "leading Republican voice at Brown", according to an X post from the New York Republicans Club.
Mr Umurzokov had moved with his family as a child from Uzbekistan to the United States, and had planned to become a neurosurgeon.
"He always lent a helping hand to anyone in need without hesitation, and was the most kind-hearted person our family knew," his family wrote in an online fundraising post.
Providence Mayor Brett Smiley said seven of the nine people injured in the shooting remained in hospital, with one in a critical condition.
He said there was "no credible, actionable, specific threat" to the area since the shooting took place, but Colonel Perez said there was an "enhanced police presence" in the city.
