Two 10-year-old boys were killed and four other people were injured when a car fleeing a traffic stop crashed in southern Wisconsin, police said.

The Waukesha County Sheriff’s Department said a deputy stopped the car for speeding and driving without license plates just before midnight Wednesday, but the driver sped away with five passengers in the vehicle.

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A police pursuit involving two deputies began in Pewaukee and ended miles away when the car was involved in a rollover crash in Delafield, about 25 miles west of Milwaukee, police said.

"The driver fled at such a high rate of speed that my deputies weren’t even in visual contact during the crash," Sheriff Eric Severson said Thursday.

Two 10-year-old boys died in the one-car crash, while two girls, ages 14 and 6, were hospitalized in critical condition, police said. A 29-year-old man who was the car's driver was in critical but stable condition and a 29-year-old woman was in stable condition.

Authorities have not released their names. But the sheriff’s department said both boys were the driver’s sons, the 29-year-old woman is the driver’s girlfriend and the 6-year-old girl is their daughter. The 14-year-old girl is the driver’s niece and all six crash victims are from Milwaukee, police said.

The pursuit and the resulting crash will be investigated by the Washington County Sheriff’s Department and the Wisconsin State Patrol.