Authorities described the incident as the worst school shooting in the United States since the Columbine massacre in 1999.
Before the shootings at the school, the suspect's grandparents were shot in their home and died later.
Six people died in the mid-afternoon Monday shooting at Red Lake High School in Minnesota, including four students.
Also killed were a teacher and a security guard, FBI spokesman Paul McCabe said.About 15 other students were injured, including two critically.
The school was evacuated after the shootings and locked down for investigation.
It was the nation's worst school shooting since two students at Columbine High School in Colorado killed 12 students and a teacher and wounded 23 before killing themselves in April 1999.