SANTA CLARITA, Calif. (AP) – Authorities scrambled Monday to find at least nine drivers who apparently escaped from vehicles trapped in a weekend tunnel inferno that killed three people on a key transportation route.

Thirty-one vehicles were involved in the pileup in the curving tunnel on Interstate 5, but the California Highway Patrol has accounted for only 23 people, including two men and a 6-year-old boy who were killed.

If all of them were at the wheel at the time of the crash, excluding the boy, that leaves nine unaccounted for drivers. The number could be higher if there were passengers in those vehicles.

Investigators are confident only three people died, but CHP Assistant Chief Warren Stanley said they have no idea what happened to the others who left their vehicles to the flames.

“We have no idea,” Stanley said. “We haven’t identified all the vehicles, we haven’t identified all the drivers.”

As of Monday, the CHP had received no missing person reports connected to the crash. Stanley said investigators expanded their search, including contacting local agencies to locate people involved.

Authorities said 10 people were hospitalized with minor or moderate injuries from the fiery crash late Friday night. Another 10 people escaped the flaming, 550-foot long tunnel unscathed.

As the highway reopened Monday, investigators worked to identify vehicles, some reduced to molten steel.

Traffic moved smoothly during rush hour after the state reopened all main lanes of the interstate. The fire-damaged tunnel, which routes trucks beneath the highway on a gentler grade down Newhall Pass, will be closed indefinitely.

“Traffic is moving wonderfully,” CHP spokesman John Lutz said. “It’s smooth and light.”

Lutz said he had been expecting the morning commute to be more congested than usual, as the truck traffic that normally takes the tunnel would be using the main freeway lanes along with cars.

The reopening of one of the area’s main roads came quicker than expected. Officials had said the freeway might remain shut for days. Lutz credited state road crews for working nonstop to reopen the freeway.

Meanwhile, commuter train operator Metrolink started running nonstop service Monday with extra cars between downtown Los Angeles and suburban Santa Clarita. By 6 a.m., passengers were arriving at Santa Clarita’s Newhall station.

“I thought it was an opportunity for me to try the train because even without accidents, the freeway is always packed,” first-time Metrolink rider Jose Garcia, 48, said.

Investigators were still trying Monday to determine the cause of the pile-up.

They have determined that 31 vehicles – including big rigs and one passenger vehicle – were involved in the crash 30 miles north of Los Angeles. The driver of the passenger vehicle is among those who escaped, Stanley said.

Authorities were waiting for dental records to help identify the dead, Los Angeles County Coroner’s Lt. Cheryl MacWillie said.

Two of the victims, believed to be a 38-year-old man and a 6-year-old boy, were riding together in a big rig, MacWillie said. Officials previously said the child in the truck was an infant.

Several trucks in the crash burst into flames and the fire spread from vehicle to vehicle. Flames shot nearly 100 feet in the air outside the tunnel and reached temperatures as high as 1,400 degrees.

State transit crews installed supports to buttress the tunnel’s walls and roof as investigators tried to determine the extent of the damage to the steel and concrete tunnel, officials said. There is no schedule for reopening the tunnel.

Truckers use I-5, the main West Coast interstate linking Mexico and Canada, to haul produce from the Central Valley to Southern California and to move goods north from the ports of Los Angeles and Long Beach.

The 1970s-built tunnel, with its long curve and darkness, has long been regarded by truckers as one of the most dangerous areas of the freeway. State transit authorities said the tunnel was safe as long as drivers were careful.