(Adds additional details from highway patrol)
LOS ANGELES, May 21 (Reuters) - A Los Angeles-bound passenger bus veered off a Southern California highway on Wednesday and flipped onto its side, killing four people on board and injuring 20 more, California Highway Patrol officials said.
As the bus was traveling through the town of Blythe, it encountered pipes on the road that had scattered when a semi-tractor truck hit the center median only minutes earlier, California Highway Patrol spokesman Jason Girard said.
The bus driver "saw some debris and veered to the right," Girard said.
The crash comes a little more than a month after a fiery collision between a passenger bus and a semi-tractor trailer in Orland, California, killed 10 people, including five Los Angeles-area high school students. The teenagers were on their way to tour a university campus.
The National Transportation Safety Board is investigating that crash, and in a preliminary report determined that the driver of a FedEx truck that slammed into the bus full of students had driven more than 350 miles and worked more than seven hours when he died in the collision.
On Wednesday, the bus operated by El Paso-Los Angeles Limousine Express Inc had originated in Phoenix and was on its way to Los Angeles when it landed on its side at the bottom of a shallow embankment, the California Highway Patrol said in a statement.
Four passengers were pronounced dead at the scene, and another 20 were injured, Girard said. The names and ages of the victims were not immediately released.
Three of the most seriously hurt passengers were airlifted to local California hospitals, and one to a hospital in Phoenix, he said. The driver was slightly hurt in the crash, which occurred shortly after 2 a.m. local time, Girard said. (Reporting by Alex Dobuzinskis in Los Angeles and Carey Gillam in Kansas City; Editing by Nick Zieminski and Gunna Dickson)