A high school student killed two of his grandparents and then went on a shooting rampage at his Minnesota school on Monday, killing five people and wounding as many as 15 others before killing himself, officials said.

The dead included a female teacher, a male security officer and four students of the Red Lake High School, which is on the Red Lake Indian Reservation in northern Minnesota, FBI Special Agent Paul McCabe was quoted as saying by news reports.

The school, which has about 300 students, was evacuated and locked down, he said.

The dead students were shot in one room, and 14 to 15 other students were wounded, he added.

About one hour after the shooting incident, the authorities found that the boy had killed his grandmother and grandfather, who was a retired policeman, an official of the Red Lake Fire Department was quoted as saying.

The boy used his grandfather's police-issued weapon in the rampage, he said.

This was the worst school shooting in the United States since April 1999 when two students at a high school in Colorado killed 13 people and wounded 23 others before committing suicide.