(CNN) -- Six people died and at least 34 were injured -- four of them critically -- in a crash involving a Greyhound bus and two other vehicles on a highway in Fresno, California, early Thursday, the California Highway Patrol said.

The accident occurred about 2:15 a.m. on northbound California Highway 99, highway patrol spokesman Officer Kirk Arnold said. The bus struck an overturned SUV that was in the highway's fast lane and then struck a second vehicle. All three vehicles traveled down an embankment and the bus slammed into a large eucalyptus tree, Arnold said.

"We're still trying to piece everything together," he told reporters.

An eyewitness, taxi driver Mike Coupland, told CNN affiliate KMPH-TV "at the last second" he saw the SUV on its side in the highway -- "no lights, no nothing." He said the bus did not have time to stop before striking the SUV.

The impact was "like a bomb going off," Coupland said. "Just pieces and parts everywhere. It was terrible."

The bus driver, he said, was "doing nothing wrong."

Arnold said authorities are not sure whether the SUV had just overturned before the crash or if it was previously in the road. The fatalities -- four women and two men -- included the bus driver and at least one person in the SUV, he said. Five people were dead at the scene, and one man died later.

The bus was en route from Los Angeles to Sacramento. There were conflicting reports about how many people were on board. Arnold told reporters it carried 47 people plus the driver, but Greyhound spokesman Timothy Stokes said 35 people were on board.

The bus had apparently made a stop in Fresno, Arnold said. Greyhound spokesman Timothy Stokes issued a statement saying the bus was en route to its next stop, Madera, California, when the crash occurred.

The highway's northbound lanes were closed, Arnold said. Greyhound representatives were on scene. The cause of the crash was still under investigation. "It's going to be a lengthy process," Arnold said.

Six people sustained moderate injuries in the crash, and 24 had minor injuries, he said.

Relief buses were sent to the scene to pick up passengers who were uninjured, Stokes said.