Police Chief Perry Beise said the victims - aged 8, 9 and 13 - suffered "superficial wounds" requiring stitches in the attack at Pleasantview Elementary in Sauk Rapids. No one else was hurt.
School Superintendent Bruce Watkins said the boy lashed out at the other students until an adult intervened. The incident lasted about 5 minutes and took place in a school hallway.
The incident happened about 7:15 a.m. as students were arriving for class, Watkins told the Minneapolis Star Tribune . After putting his backpack away, the second-grade student took out a knife and slashed three students with it, the superintendent said. School officials immediately called police and an ambulance service.
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Beise said the boy was interviewed by police and released to his parents. The case will be forwarded to the county prosecutor's office.
One of the injured children is a seventh-grader who was waiting to catch a transfer bus. The other two are Pleasantview Elementary students.
Watkins said the school made mental health counselors available to students and staff. Classes went on as scheduled. The attacker will not be allowed back in school, the superintendent said.
Sauk Rapids is about 70 miles (113 kilometers) northwest of Minneapolis. Pleasantview has 720 students in kindergarten through fifth grade, according to the school district's website.
Weapons in schools and school security have been ongoing issues in the U.S.
A 19-year-old former student at Marjory Stoneman Douglas High School in Parkland, Florida, is accused of bringing an assault rifle to the school in February and killing 17 students and staff members. In December 2012, Adam Lanza, 20, shot his mother to death at their Newtown, Connecticut, home before killing 20 first-graders and six educators at Sandy Hook Elementary School. Lanza killed himself as police arrived at the school.
Police in Sauk Rapids haven't returned a phone message seeking comment. In a news release, police said the student "randomly cut" three students and that the injuries were minor.
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