A Minnesota teenager killed his grandfather, who was a police officer, then strapped on the older man's bulletproof vest and drove his police cruiser to a school on a shooting rampage that left 10 dead, the FBI says.

FBI agent Michael Tabman said 16-year-old Jeff Weise appeared to have been acting alone when he killed nine people and then himself in Monday's massacre on the Red Lake reservation, about 120 kilometres south of the Canadian border.

"At this time, we do believe he acted alone, but we have to explore all possibilities," Tabman said at a news conference early Tuesday afternoon.

Jeff Weise

"The nature of the activities would indicate there was some planning."

No motive has yet been uncovered and Tabman said he couldn't confirm whether Weise was the same person who posted messages on a neo-Nazi website months earlier using the nickname "Todesengel" – German for "angel of death."

FBI agent Michael Tabman

No 'hit list' found: FBI

Tabman said police haven't found a "hit list" and Weise's targets appeared to have been random, apart from his first victims – the teen's grandfather, Daryl Lussier, and Lussier's wife, Michelle Sigana.

After shooting them with a .22-calibre handgun, he donned a bulletproof vest and police belt that belonged to his grandfather, who was a sergeant with the tribal police, and drove his cruiser to the school, Tabman said.

He shot and killed an unarmed 28-year-old security guard stationed at a metal detector near the school's entrance who tried to stop him.

He stalked the halls, killing a teacher and five students in a classroom and then wandering about "firing randomly," Tabman said.

When four police officers pursued him through the building, he shot at them and at least one officer returned fire.

Tabman said it's not yet known whether they managed to hit the teen, who returned to the room filled with his victims' bodies and killed himself.

Witnesses said Weise smiled and waved at some students after killing the guard and at one point asked a student if he believed in God before shooting him fatally.

That was the same question that teenaged gunmen allegedly asked students at Columbine high school in Littleton, Colo., during a shooting in April 1999.

Monday's incident is the worst school shooting in the United States since the Columbine tragedy, which left 12 students and a teacher dead, as well as the two gunmen.

5 students remain in hospital

Five teenagers remained in hospital Tuesday, two of them with critical injuries, and about 14 people in total were wounded.

Some victims were shot at close range and one student later died from a head wound, hospital officials said.

Two other students who were shot in the head and face remain in critical condition and have been airlifted to a hospital in Fargo, N.D.

Another teen got a bullet through the hip and two students were shot in the chest, but aren't in critical condition.

"I think there was an intent to kill," Tim Hall, the hospital's emergency nursing director, said at a morning news conference.

Teen suspended from school

Weise's relatives and others in the community have described him as a loner with a difficult past.

They told the St. Paul Pioneer Press that the teenager's father committed suicide several years ago and his mother had been living in a nursing home since suffering brain injuries in a car accident.

He had been a student at the school but was recently placed in a learn-at-home program because of a rule infraction that school officials would not specify Tuesday.

Complaints about school on neo-Nazi website

As well, someone identifying himself as "Jeff Weise, a Native American from the Red Lake Indian Reservation in Minnesota" posted messages on a neo-Nazi website, using the name Todesengel.

In the messages, which began in 2004, Todesengel writes, "I'm being blamed for a threat on the school I attend because someone said they were going to shoot up the school on 4/20, Hitler's birthday."

Later, the same writer says he was cleared as a suspect. He also complains several times about his fellow students and teachers, dismissing them as small-minded because they disagreed with his National Socialist views on racial purity.

The high school on the Red Lake Indian Reservation, about 400 kilometres north of Minneapolis, has about 300 students.

The reservation is home to about 5,000 Ojibwa Indians, who are also called Chippewa. The community operates three casinos and other tourist attractions.