A Colorado State Patrol trooper was on the shoulder of Interstate 25 near Castle Rock when a tractor-trailer rig crossed the solid white line and hit him, killing him instantly, according to court documents obtained by 9NEWS.

Trooper Cody Donahue, an 11-year veteran, was out of his patrol car investigating another crash when he was killed shortly before 2 p.m. Friday.

Noe Gamez-Ruiz, 41, the driver of the truck, was arrested on suspicion of careless driving causing death and failure to yield the right of way to an emergency vehicle.