After killing two of his grandparents, a US student on Monday staged a shooting rampage at his Minnesota high school, resulting in six deaths including himself, officials said.

　　Among the dead was a male security guard, a female teacher, three students and the gunman, FBI agent Paul McCabe said. As many as 15 other students were injured.

　　"We believe that one of those students is the shooter," McCabe said. The school was evacuated and locked down, he said.

　　"At this time, we believe he was acting alone," McCabe said. He would not comment on a possible motive, saying, "It’s far too early in the investigation." The slain students were shot in one room, he added.

　　Authorities discovered about an hour later that the boy had shot and killed his grandmother and grandfather who was a longtime police officer on the reservation.

　　The shootings occurred about 3 p.m. (4 p.m. ET), in Red Lake High School, a school of 300 student that is on a sovereign Indian reservation within Beltrami County, about 25 miles north of Bemidji, a town of about 25,000 residents, many of them Ojibway Indians, he said.