EAST LANSING, MICH. -- The 43-year-old gunman who killed three students and wounded five others at Michigan State University had no apparent connection to the campus, police said Tuesday as they searched for a motive for shootings that terrified the community for hours.

Investigators were sorting out why Anthony McRae fired inside an academic building and the student union just before 8:30 p.m. Monday. An hourslong lockdown at the campus in East Lansing ended when he killed himself miles away while being confronted by police.

The shooting happened the day before the fifth anniversary of the Parkland, Florida, school shooting that killed 17 and is the latest in what has become a deadly new year in the U.S.

"We have to do something to stop the gun violence that's ripping apart our communities," President Joe Biden said in a speech Tuesday, mentioning Michigan State.

Meanwhile, a school district in Ewing Township, New Jersey, closed for the day after investigators said that McRae, who lived in the area years ago, had a note in his pocket indicating a threat to schools there. But it was determined there was no credible threat, local police said later in a statement shared by the superintendent.

The dead and injured in the gunfire at Berkey Hall and the MSU Union, a popular place to eat and study, were all Michigan State students. Five remained in critical condition at Sparrow Hospital, said Dr. Denny Martin, who fought back tears during a news conference Tuesday.

"We have absolutely no idea what the motive was," said Chris Rozman, deputy chief of campus police, adding that McRae, of Lansing, was not a student or Michigan State employee.

"This is still fluid," Rozman said. "There are still crime scenes that are being processed, and we still are in the process of putting together the pieces to try to understand what happened."

The dead were all from the Detroit area. Two gradated from separate high schools in the Grosse Pointe district: Brian Fraser, president of Phi Delta Theta fraternity, and Arielle Anderson. Alexandria Verner, a graduate of Clawson High School in another Detroit suburb, also died.

"If you knew her, you loved her and we will forever remember the lasting impact she has had on all of us," Clawson Superintendent Billy Shellenbarger said in an email to families.

The shootings took place in an area of older, stately buildings on the northern edge of the Michigan State campus, one of the nation's largest at 5,200 acres. Just across busy Grand River Avenue lies East Lansing's downtown, teeming with restaurants, bars and shops.

"Our Spartan community is reeling today," Gov. Gretchen Whitmer, a Michigan State graduate, said at the morning briefing.

Biden pledged his support during a phone call, she said.

"We mourn the loss of beautiful souls today and pray for those who are continuing to fight for their lives. ... Another place that is supposed to be about community and togetherness is shattered by bullets and bloodshed," Whitmer said.

Michigan State has about 50,000 students, including 19,000 who live on campus. As hundreds of officers scoured the campus, about 90 miles (145 kilometres) northwest of Detroit, students hid where they could Monday night.

At 11 p.m., police were still searching for McRae when he turned up on school security cameras, and his image was quickly released to news media. An "alert citizen" saw the picture, recognized him in the Lansing area and contacted police within minutes, Rozman said.

"That was exactly what we were trying to achieve by releasing that picture. We had no idea where he was at that point," the deputy chief said.

Officers confronted McRae about 5 miles (8 kilometers) from campus in an industrial area, where he killed himself, Rozman said.

In 2019, McRae was accused of illegally possessing a concealed weapon, according to the state Corrections Department, but pleaded guilty to having a loaded gun in a vehicle, a misdemeanor. He completed 18 months of probation.

A large police presence was in his Lansing neighbourhood overnight. Suzanne Shook said she has lived a block away from McRae for about a year.

"We never spoke to him," Shook said. "When he would be walking or riding his bike, he was always straightforward and wouldn't look at anybody."

Students, meanwhile, recalled the previous night's terror. Dominik Molotky said he was learning about Cuban history around 8:15 p.m. when he and the other students heard a gunshot outside the classroom. He told ABC's "Good Morning America" that a few seconds later, the gunman entered and fired three to four more rounds.

"I was ducking and covering, and the same with the rest of the students," Molotky said.

Claire Papoulias, a sophomore, described on NBC's "Today" how she and other students scrambled to escape a history class through a window after the gunman entered from a back door and began firing.

"As soon as I fell out of the window I kind of hit the ground a little. I just grabbed my backpack and my phone, and I remember I just ran for my life," she said.

All classes, sports and other activities were cancelled for 48 hours. Interim university President Teresa Woodruff said it would be a time "to think and grieve and come together."

Dozens of people have died in mass shootings so far in 2023, most notably in California, where 11 people were killed as they welcomed the Lunar New Year at a dance hall popular with older Asian Americans. In 2022, more than 600 mass shootings occurred in the U.S. in which at least four people were killed or wounded, according to the Gun Violence Archive.

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Associated Press writers Ed White and Corey Williams in Detroit contributed to this report.