(Reuters) - Three girls scouts and a woman were killed in Wisconsin on Saturday when a pickup truck veered off a road and hit them while they were picking up litter, police said.

Another girl scout was critically injured when the black Ford F-150 pickup veered off the road and hit them on a two-lane county road near Lake Hallie, about 90 miles (145 km) east of Minneapolis, police said in a statement.

The driver, Colton Treu, 21, of nearby Chippewa Falls, Wisconsin, later turned himself in to police, according to the statement.

Police did not indicate what may have caused Treu to veer off the road.

The girls were in the fourth grade at Halmstad Elementary School in Chippewa Falls, a relative of an uninjured girl told the Minneapolis Star Tribune newspaper. They were all wearing bright safety vests and were accompanied by several adults.

Two of the girls and the woman were pronounced dead at the scene and a third was taken to a hospital, where she later died, police said. The fourth girl was hospitalized.