A crash involving an RTA express bus, a box truck and several other vehicles injured seven -- one of them critically -- and snarled traffic on southbound Interstate 15 for hours, the CHP reported.

The crash on I-15, just north of Winchester Road, was reported at 7:50 a.m., according to the California Highway Patrol.

Callers reported that the semi overturned, as did another vehicle in the middle lane.

According to the California Highway Patrol, "the front of the RTA bus collided with the rear of the Isuzu box truck. This impact caused the box truck to be pushed forward into the rear of the Nissan pickup," a statement released by the CHP read.

"The box truck then veered across the right shoulder and overturned on the descending embankment. The Nissan pickup was pushed forward and the front of the Nissan collided with the rear of the Chevrolet SUV," the statement read.

The bus eventually went all across the lanes and slammed into a median wall.

The RTA bus involved in the accident was from CommuterLink express Route 208, which travels from Temecula to Riverside. The route is operated by RTA’s contracted service provider, Empire Transportation, according to the RTA.

A chain reaction of vehicles led to the injury of at least six people.

The crash immediately backed up traffic north to Murrieta Hot Springs Road and a SigAlert was quickly issued to warn motorists of slowed speeds, the CHP reported.

Traffic was also backed up on Interstate 215 to Newport Road, according to the CHP.

The bus driver had his leg partially severed, the CHP reported.

Five people on the bus suffered mild to moderate injuries and were taken to an area hospital.

Cal Fire reported seven patients -- one with critical; two with moderate and four with minor injuries -- were taken to area hospitals, one of which has a trauma center.

Lanes were reopened by 10:30 a.m., though the SigAlert for slow speeds remained in effect for a time.

The SigAlert was canceled shortly before noon, according to the CHP.

The cause of the crash was under investigation and video footage was being examined, authorities said.