SAN JOSE — Sleeping passengers on a Greyhound bus awoke to a freeway nightmare Tuesday morning when the vehicle overturned on a rain-drenched Highway 101 in South San Jose, killing two women and injuring at least six others.

Potentially making the situation more tragic is a passenger’s claim that the driver of the bus, who admitted to fatigue, was dozing off in the moments leading up to the crash in the northbound lanes approaching the flyover connector to Highway 85. The bus left Los Angeles on Monday night and was heading to stops in downtown San Jose and San Francisco en route to Oakland.

“Everything is under investigation,” Greyhound spokeswoman Lanesha Gipson said. “We’re investigating everything that took place between when the bus left Los Angeles and when the collision occurred.”

The CHP late Tuesday identified the bus driver as 58-year-old Gary Bonslater, who suffered moderate injuries.

The crash occurred around 6:40 a.m., amid heavy rain and just as commuter traffic was building on one of the Bay Area’s busiest roadways. For a time, three of the four lanes in both directions on 101 were shut down, backing up traffic for almost nine miles to Cochrane Road in Morgan Hill, a Caltrans spokeswoman said.

It was the worst incident by far in a storm-snarled morning commute across the Bay Area, where nearly 480 crashes were reported between midnight and noon Tuesday, according to CHP spokesman Daniel Hill.

A witness trailing the bus before the crash told California Highway Patrol investigators that about 200 feet before the flyover ramp, the bus changed into the carpool lane, lost traction, got briefly airborne, and landed on its right side.

“It sounded like a lightning strike hit the bus,” passenger Alex Ehlers, of Denver, told KCBS and other assembled media. “Then I just heard people yelling, people screaming, smoke coming from the rear of the bus.”

Ehlers also recalled a “loud scraping sound” for 10 to 15 seconds, followed by a feeling of being “weightless” and then the bus “seesawing” on the center median. When he climbed out with his girlfriend, he described a grisly scene.

“There were people lying face down in the concrete,” Ehlers said.

CHP Sgt. Lisa Brazil said passengers on the bus told investigators that most of the people on board were sleeping at the time of the crash. Ehlers said the driver also appeared to be nodding off at the wheel.

“About 10 miles earlier, the bus driver pulled over to catch himself, and was unable to,” he said. “I could feel him weaving and jerking the wheel a little bit, and I knew that it just wasn’t going to end right.”

The CHP later said that the driver told investigators that he was fatigued before the crash and got coffee at a stop in Gilroy. The driver reportedly also told the CHP that he remembered hitting crash barrels on the roadway and then the bus being on its side.

Gipson said she could not comment on the fatigue claim about the driver but asserted that Greyhound drivers are required to have at least nine hours of sleep before a trip, which is one hour more than federal recommendations for any 10-hour driving period.

The ensuing crash investigation is expected to involve Greyhound as well as the California Highway Patrol’s Multidisciplinary Accident Investigation Team (MAIT), which Brazil said was summoned to the area from San Luis Obispo. MAIT investigators will compile a 24-hour profile of the driver and examine the bus for any mechanical problems.

The National Transportation Safety Board is also sending a federal Highway Investigation Team to run a parallel safety investigation, agency spokesman Eric Weiss said.

“We’re looking at all types of survival factors, such as fatigue, structural issues, the whole gamut,” Weiss said. “What we can learn from this accident that will help prevent other incidents like this in the future.”

San Jose Fire Capt. Christopher Salcido, a department spokesman, said upward of 40 firefighters responded to the scene and triaged at the site as part of a “multi-patient, potentially multi-casualty” scenario.

“A bus like this, resting on a median, is a large incident,” Salcido said. “Our first thing is to get people out, those that aren’t injured, so we can start treating those that are and those patients that need to be extricated.”

Early Tuesday afternoon, the two women killed were removed from the scene. At least six passengers were taken to area hospitals, one with serious injuries and the others, including an 8-year-old boy and the driver, with minor to moderate injuries.

One of the women killed was identified as 51-year-old San Francisco resident Fely Olivera, a friend of Olivera’s son told ABC7 on Tuesday afternoon. The friend also told the station that Olivera, a recent immigrant from the Philippines, traveled to Los Angeles to visit her two sons and headed back to San Francisco early to make a doctor’s appointment.

Gipson said the bus was occupied by 20 customers and the driver. The bus was cleared from the scene and all lanes were reopened about 5 p.m. Tuesday.

At least one of the passengers was taken to nearby Kaiser Permanente San Jose, and hospital spokesman Karl Sonkin said the person was in stable condition. Others were taken to Regional Medical Center of San Jose, Valley Medical Center, and Good Samaritan Hospital.

Passengers who were not injured or did not need extensive medical treatment were picked up by another bus and were expected to be shuttled toward the next stop, Gipson said. She also said that friends and family of anyone who was on bus schedule no. 6876 can call 1-800-972-4583 to check on their status.

Bus crash fatalities in California have been on a sharp uptick over the past few years. According to the National Highway Traffic Safety Administration, the state saw four bus deaths in 2010, one in 2011, and none in 2012 before recording 11 in 2013 and 17 in 2014. Data for 2015 was not available from the agency.

Contact Mark Gomez at 408-920-5869 and follow him at Twitter.com/MarkMgomez.