RED LAKE, Minn. – Authorities were trying to determine yesterday what caused a teenager to gun down his grandfather, put on the man’s police-issue belt and bulletproof vest, and drive his marked squad car to a high school, where he began shooting his classmates at will.

Jeff Weise, whom authorities said was 16 or 17, killed nine people and wounded seven Monday before trading gunfire with a police officer and apparently shooting himself. His motive still wasn’t clear yesterday, but the FBI said the shootings appeared to have been planned in advance.

It was the nation’s worst school shooting since the Columbine massacre in April 1999 that ended with the deaths of 12 students, a teacher, and the two teen gunmen.

The killings on this northern Minnesota Indian reservation began at the home of Weise’s grandfather, Daryl Lussier, 58, who was shot to death with a .22-caliber gun, according to Michael Tabman, the FBI’s special agent in charge for Minneapolis. Also killed was Lussier’s companion, Michelle Sigana.

Lussier had worked as a tribal police officer for decades. Weise then drove the older man’s squad car to the school, where he gunned down security guard Derrick Brun at the door and spent about 10 minutes inside, targeting people at random.

Hearing the shots, students, and adults barricaded themselves into offices and classrooms and crouched under desks, authorities said. Five students were shot to death and two 15-year-olds remained in critical condition at a Fargo, N.D., hospital with gunshot wounds to the face.

“Right now, we are in utter disbelief and shock,” said Floyd Jourdain Jr., chairman of the Red Lake Band of Chippewa.

Authorities were investigating whether Weise, who dressed in black and wrote stories about zombies, may have posted messages on a neo-Nazi Web site expressing admiration for Adolf Hitler.

Using the handle Todesengel – German for “Angel of Death” – the writer identified himself as Jeff Weise of the Red Lake Reservation. In April 2004, he referenced being accused of “a threat on the school I attend,” though it says he was later cleared.

Mr. Tabman said yesterday he couldn’t confirm whether Weise was the person who made the postings.

School board member Kathryn Beaulieu said Weise had been placed in the school’s Homebound program for a policy violation. She did not elaborate.

Relatives told the St. Paul Pioneer Press that Weise was a loner who was often teased by others. They said his father committed suicide four years ago and his mother was living in a Minneapolis nursing home because she suffered brain injuries in a car accident.

Residents at the home where Weise was believed to have lived with his grandmother declined to comment yesterday. School was canceled for the day as investigators scoured the building for clues.

At the Capitol in St. Paul, several hundred people attended a prayer ceremony for victims of the shooting where religious leaders joined Indians in drumming and chanting.

“Once again, our people have been hit … but our people are strong,” said Ona Kingbird, a Red Lake tribal member. “We’ll come out of it.”

The governor of Minnesota, Tim Pawlenty, a Republican, expressed his condolences for the families of the victims and said it appeared the school had “very rigorous security.”