A manhunt is under way after two students were killed and nine other people were injured in a shooting at Brown University in Rhode Island.
Officials say the gunman opened fire in a classroom at around 16:00 local time (21:00 GMT) on Saturday at the Barus and Holley engineering building on the eastern end of the Ivy League university's campus, where final exams were taking place.
In a Saturday evening news briefing, Providence Mayor Brett Smiley said most of the injured are in a "critical but stable" condition in the locked-down Rhode Island Hospital but warned the number of casualties could rise.
A shelter in place warning remains in place for the greater Brown University area.
The identities of those killed or injured have not yet been released by officials.
"This is a day that we hoped never would come to our community. It is deeply devastating for all of us," said Brown University President Christina Paxson in a statement.
Police have released limited information about the suspect. However, they have provided CCTV footage of him walking out of the building after the shooting.
Neither the suspect's face nor a weapon are visible in the video.
Providence Deputy Police Chief Tim O'Hara said the suspect is a male who was dressed all in black and appeared be in his thirties.
"We're utilising every resource possible to find this suspect," he said.
Barus and Holley engineering building is a seven-story building that includes 117 laboratories, dozens of classrooms and three lecture halls.
An economics professor told local public media outlet Ocean State Radio that the shooting had taken place during a review session for her course, which was led by her teaching assistant.
"He said that the shooter came in the doors, yelled something - he couldn't remember what he yelled - and started shooting," Rachel Friedberg said.
"Students started to scramble to try to get away from the shooter, trying to get lower down in the stadium seating, and people got shot," she added.
Officials cleared the building on Saturday afternoon but found neither the suspect nor a weapon.
They said the gunman used a firearm but did not have information about what kind.
Residents in the greater Brown University area have been told to stay at home and stay inside, or to stay away until the shelter-in-place is lifted.
In a statement, the university said law enforcement agencies would enter non-residential buildings to escort people to safe locations. Other people in residential buildings should continue to shelter in place.
Steph Machado, a reporter for the Boston Globe, told BBC News that restaurants around the campus have locked their doors with staff and customers waiting inside until the emergency order is lifted.
"There are flashing lights everywhere," she said.
The scramble in the wake of the shooting by officials was clear as the university reported that a suspect had been taken into custody on Saturday afternoon before retracting the information shortly after.
Smiley confirmed the police had briefly held someone who after questioning was found to have had no involvement in the attack.
Mari Camara, 20, a Brown University student from New York City, told the Associated Press that she was coming out of the library and rushed inside a restaurant to seek shelter during the shooting. She spent the next three hours hiding there.
"Everyone is the same as me, shocked and terrified that something like this happened," Camara said.
University exams scheduled for Saturday have been cancelled, provost Frank Doyle said.
President Donald Trump, speaking to reporters as he returned to the White House from attending the annual Army and Navy football game, described the shooting as "a terrible thing".
"All we can do right now is pray for the victims and for those that were very badly hurt," he said.
Rhode Island Governor Dan McKee said in a statement: "Our capital city experienced an unthinkable tragedy today. Our hearts are with the people of Providence and all those impacted."
Brown University, one of the one of the oldest higher education institutions in the US, is part of the Ivy League, a group of elite universities in the northeast of the country.
The university, which has more than 11,000 students, is located in Providence, Rhode Island's capital city, located about 50 miles (80km) from Boston and 180 miles (290km) from New York City.
The attack on the campus brings the number of mass shootings in the US to 389 for this year, according to the independent analysis website Gun Violence Archive (GVA).
It defines mass shootings as having four or more victims killed or injured, not including the attacker.
