A passenger bus carrying 32 passengers flipped onto its side and slid down an embankment after trying to avoid steel pipes that spilled from a big rig early Wednesday morning. The crash killed four and sent 21 others to the hospital with injuries.

The crash happened at about 2:15 a.m. Wednesday morning near Blythe, California, which is near the Arizona border. A tractor trailer was traveling east when it hit the median and jackknifed, spilling metal pipes into all of the lanes in both directions on Interstate 10, according to Terri Kasinga of the California Department of Transportation.

The bus was traveling westbound when it encountered the spilled pipes. It either hit one of the pipes or tried to avoid them when it skidded into the shoulder and then down the 50-foot embankment.

Besides the bus, four other vehicles crashed in the eastbound lanes. Unknown injuries were reported in those crashes, but no one was killed.

"The pipes subsequently caused some of the vehicles to crash into them in the eastbound lanes and it appears they could have rolled into westbound traffic which is where the bus was traveling," Kasinga said.

Helicopters transported critically injured passengers from the bus to hospitals. 

The crash caused lane closures in both directions of the interstate. Lanes will be closed until at least Wednesday afternoon and travelers in the area are encouraged to find alternate routes, Kasinga said.

The bus, owned and operated by El Paso-Los Angeles Limousine Express Inc., is a low-cost operator that transports passengers between major cities in the Southwest. The bus was traveling from El Paso to Los Angeles. It made a stop in Phoenix to switch drivers.

Blythe is about 150 miles west of Phoenix and over 200 miles east of Los Angeles. With delays on this main highway expected to last into the afternoon, commuters can expect lengthy delays and slow traveling in the area.

Representatives from the bus company have not yet responded to requests for comments about the crash.