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Swedish police say all three people who were killed when a bus skidded off the road and crashed Sunday in central Sweden were high school students.

Police spokesman Peder Jonsson declined to provide more information on the students Monday.

Authorities say the two-story bus was carrying 52 students, six adults and a driver when it crashed into a ditch and fell on its side on a main highway about 7 a.m. — nine hours after it had left the southwestern town of Skene on a school trip to a northern ski resort.

Ostersund hospital spokeswoman Anna-Lena Hogstrom said Monday that 11 people from the accident were still being treated, including two with serious but not "life-threatening" injuries.

Jonsson said police were investigating but did not know the cause of the accident.