A sweeping manhunt to locate the suspect who carried out a mass shooting at Brown University is stretching into its third day.

The shooter gunned down students studying for finals, killing two and injuring nine in Providence, Rhode Island, on Saturday afternoon. Authorities initially detained a person of interest but revealed late Sunday that they are releasing the 24-year-old man due to a lack of evidence, meaning the suspect remains at large.

More than 400 law enforcement officers, including FBI and ATF agents, are searching for the suspect. Authorities have struggled to identify a male dressed in all black, based on university video footage that did not appear to capture a face. The man is believed to have fled on foot.

Rhode Island Attorney General Peter Neronha had a tense exchange with reporters on Sunday when one questioned why there were not enough cameras at Brown University to catch the gunman.

“There just weren’t a lot of cameras in that Brown building, is the reality,” Neronha said. “I mean, we’re not holding back a video that we think would be useful. And I don’t think I should even have to say it. … I don’t know what to tell you, but we’re going to go out and find whatever evidence we can to solve this case as quickly as we can.”

Federal law enforcement officers initially detained Benjamin Erickson, a 24-year-old Army veteran from Wisconsin who appeared to hold ties to Washington, D.C., as the primary person of interest in the mass shooting.

After Erickson was detained at a hotel in Coventry, Rhode Island, overnight, Providence Police Chief Oscar Perez said Sunday that authorities were confident he was the gunman and charges could be filed soon.

But later that day, the investigation took a twist when officials announced that Erickson would be released, with Neronha revealing that evidence “now points in a different direction.” Mayor Brett Smiley said they are looking for “additional video evidence” as the primary university surveillance footage did not appear to reveal his face.

The tip that led to the 24-year-old being detained was fielded by the FBI, Perez said late Sunday.

“It was actually picked up by the FBI, and they followed through with it, and they ended up coming and locating this individual of interest,” Perez said.

Gov. Daniel McKee (D-RI) said he regretted that Erickson faced so much attention and that the investigation would proceed more carefully.

“That was really unfortunate,” he said. “It’s hard to put that back in the bottle.”

Authorities must now review all the evidence, former FBI Special Agent Steve Moore told CNN’s Rosemary Church.

“No investigation is perfect, and you’re going to go through ups and downs,” he said after the “error” was made in detaining Erickson. “So you pretty much have to throw out, not the evidence, but the conclusions from the evidence. … And you start, basically, as if you’re seeing the evidence for the first time and say, ‘Where should this lead us?’”

Brown University’s Department of Public Safety has more than doubled its staffing following the deadly shooting, according to the school.

Colleges must balance the need for security with the educational mission, Rob Kilfoyle, president of the International Association of Campus Law Enforcement Administrators and director of public safety and emergency management at Humber Polytechnic in Toronto, told the Washington Post.

“That’s probably one of the toughest things that we have to do in campus public safety, is find that equilibrium between not wanting it to seem oppressive and too restrictive, but also providing sufficient security,” he said.