(CBS) — A New Mexico hospital increased the death toll to six from a crash involving a Greyhound bus and tractor trailer. The accident occured along Interstate 40 in New Mexico, near the Arizona border.

Investigators believe the tractor trailer crossed the median and hit the bus head on. The front end of the bus was ripped off and debris scattered across the highway as emergency crews desperately tried to reach victims.

The bus was carrying 49 passengers and heading to Phoenix, Arizona. Many of the seriously injured people were taken to an area hospital. One trauma center reported getting six patients, with three in critical condition.

“We are fully cooperating with local authorities and will also complete an investigation of our own,” Greyhound spokeswoman Crystal Booker said in a statement.

The crash occurred near the town of Thoreau. It forced the closure of the westbound lanes of the interstate and traffic was backing up as travelers were diverted.

Mireya Villarreal contributed to this report.