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

A “grinning and waving” high school pupil went on a shooting rampage on an Indian reservation, killing his grandparents at their home and then seven people at his Minnesota school.

A 14-year-old boy said to have been shot in the forehead by a fellow high school pupil has died, Minnesota hospital officials said today.

A 15-year-old boy opened fire at his high school, killing one pupil and critically wounding a second before a teacher apparently talked him into dropping the gun.