FLAGSTAFF, Ariz. -- An Arizona Department of Public Safety officer remains in critical condition after he was hit by an out-of-control tractor-trailer on Interstate 17 near Munds Park and south of Flagstaff.

Kenneth Henscheid had just arrived at the scene of a rollover accident involving a van and had parked his patrol car on the northbound right shoulder of I-17 at milepost 322, DPS investigators said. The driver of the van was originally northbound but had lost control, crossing through the median and rolling over into the left southbound lane, DPS said.

Henscheid was checking on the passengers of the van when he and the van were struck by the northbound tractor-trailer, DPS officers said.

The impact threw Henscheid under a third vehicle that had just pulled over on the right southbound shoulder to avoid being struck by the tractor-trailer, pinning Henscheid to the ground, DPS said. Several motorists lifted the car off of him and pulled him free, DPS said.

One person was able to use the officer's police radio to call the DPS dispatch center in Flagstaff for help, according to DPS investigators.

Henscheid suffered multiple fractures and internal injuries, DPS officials said.

DPS officers said a preliminary investigation suggests the bridge just south of the collision had iced over, contributing to the initial van rollover and the tractor-trailer losing control.

Henscheid, 40, originally of Phoenix, is a 12-year veteran of DPS.