STERLING — Two passengers aboard an ambulance died when it slammed into the rear of a semitrailer on Interstate 76 near here, according to the Colorado State Patrol.

The patient being transported by the ambulance, Kelsey Schlichenmayer of Burlington, survived, Trooper Eric Wynn said in a statement.

Killed in the crash were Karen Woods, 43, of Elizabeth, and Vicky Thomas, whose hometown was not immediately known, Wynn said. Both were pronounced dead at the scene.

The accident happened at about 4 p.m., 15 miles west of Sterling, forcing the closure of the interstate's westbound lanes for several hours.

The 22-year-old driver of the ambulance, Chris Larusso of Westminster, and passenger Dan Beza, 31, of Centennial, were both transported to Sterling Regional Medical Center with unknown injuries, Wynn said.

Semitrailer driver Plamen Zelyamov of Chicago was not injured.

The crash remained under investigation.