SANTA MARIA, California (CNN) -- Driver fatigue likely contributed to the crash of a Greyhound bus Sunday morning that left two people dead, a spokesman for the California Highway Patrol said."There's a strong possibility that the driver fell asleep and ran off the road," said California Highway Patrol Cmdr. Dan Minor.The bus, carrying the driver and 43 passengers, was on its way from Los Angeles to San Francisco.It drifted off the right side of U.S. Highway 101, went down a 10-15 foot embankment and skidded about 100 yards, finally coming to rest on its side against a eucalyptus tree, said Capt. Keith Cullom of the Santa Barbara County Fire Department."So the people inside the bus had been tossed around significantly," Cullom said. No other vehicles were involved in the accident, he said.One of the dead was 23-year-old Martha Contreras, who was seven months pregnant, Minor said. Her fetus also died.Minor predicted that the death of the fetus would eventually be counted in the death toll.The other fatality was Faro Jahani, a 50-year-old man, Minor said.All of the other passengers were injured, including seven with serious injuries and four with lighter injuries, Minor said. Most had bumps and bruises and were treated and released from nearby hospitals.Witnesses: Bus suddenly driftedA man driving on the highway around the time of the 7:10 a.m. (10:10 a.m. ET) accident said the bus drove right in front of him."(The) bus came up behind me, came around me, passed me, and then came into the slow lane ... instead of straightening up in the slow lane he kept on going over," said Manuel Bonilla. "As soon as he hit the white fog line I knew he was going to go over the edge."Minor said witnesses reported that the bus "suddenly drifted to the right without signaling or braking."He said there was no indication of alcohol or drug impairment in the driver, only fatigue. He identified the driver as Samuel Bishop. Bishop, 63, had apparently been called in to work Saturday night from his home in Fresno, Minor said. He drove a route from Fresno to Los Angeles beginning at 8 p.m. -- 220 miles -- and then began the second route from Los Angeles.He drove 158 miles to near Santa Maria. His shift would have ended in San Luis Obispo, just 32 miles north of Santa Maria.Minor said it's likely Bishop will be charged with misdemeanor vehicular manslaughter.Passenger Valerie Bradley said everyone on the bus was asleep when the crash happened."I heard a 'boom,' then tip over, then glass shatter everywhere," Bradley said.Other passengers injuredShe said the driver said he had chest pains when emergency officials arrived on the scene, but he was able to kick out the glass door on the bus and get himself out.Bradley was angry that there were no seatbelts for the passengers, but there was one for the driver.Another passenger, Carole Pitts, said after the crash she saw the driver suspended sideways from his seatbelt.Some of the other victims had to be cut from the wreckage with hydraulic tools, officials said.Helicopters landed on the freeway to transport the injured to Marion Hospital in Santa Maria and Cottage Hospital in Santa Barbara, according to Santa Barbara County Fire Department spokesman Kent Boisen.Two other Greyhound buses went to the scene to take people to their final destinations, said Greyhound spokeswoman Kim Plaskett from Dallas.People concerned that a friend or relative may have been on the bus can call Greyhound's hot line at 1-800-972-4583.CNN's Kareen Wynter contributed to this report.