Providence Police Chief Oscar Perez said earlier at a news conference on Sunday that a man in his 20s was taken into custody in connection with Saturday's gun violence but gave no further details.
But at a late-night news conference hours later, Providence Mayor Brett Smiley and other state and local officials told reporters the man detained would be released from custody, saying the investigation was going into a "different direction".
"We have not yet solved this case, but I am confident we are going to do that in the near future," Rhode Island Attorney-General Peter Neronha said.
Officials declined to elaborate on why the man who was taken into custody was detained in the first place.
"There was a quantum of evidence which justified detaining this person as a person of interest," Neronha said.
Authorities said they believe an unidentified person pictured in surveillance footage to be the person they were still looking for.
FBI Director Kash Patel said earlier Sunday in a post on X that the person of interest had been detained in a hotel room in the Rhode Island town of Coventry, a 30-minute drive from the Brown campus.
An FBI team specialising in cellular data analysis used geolocation information to track the suspect, Patel said.
The mass shooting - the latest of nearly 400 in the US this year, according to the Gun Violence Archive - shook the community at the university, one of the oldest in the United States.
The school cancelled exams, and classes, for the rest of the year and the campus was quiet on Sunday as a light snowfall blanketed the city.
Providence Mayor Brett Smiley said that authorities, as of midday on Sunday, had not yet contacted all of the victims' family members because some were travelling.
He invited residents to a previously planned event on Sunday to light a Christmas tree and a menorah to mark the first night of Hanukkah.
"It is quite clear that if we can come together as a community and shine a little bit of light tonight, I think there's nothing better that we could be doing," Smiley said.
Seven people injured at Brown University were in stable condition, Smiley said. One remained in critical but stable condition, while another had been discharged, he added.
Shelter-in-place orders at the university and nearby areas were lifted on Sunday. Smiley said earlier in the day that residents should expect a visible police presence across the city.
The gunman fled after shooting students in a classroom in Brown's Barus & Holley engineering and physics building, where outer doors had been left unlocked while exams were taking place, officials said on Saturday.
Authorities on Saturday released a short video clip of a person of interest dressed in black walking near the engineering building.
Providence Deputy Police Chief Timothy O'Hara said on Saturday the individual may have worn a mask, but officials were not certain.
Brown President Christina Paxson told reporters that all or nearly all of the victims were students, adding: "This is the day one hopes never happens, and it has."
