Driver of California Tour Bus May Not Have Braked Before Hitting Tractor-Trailer, Officials Say

— -- The driver of the California tour bus that killed 13 passengers and injured 31 others Sunday may not have stepped on the brakes before the bus plowed into another vehicle, officials said today.

There was no indication of braking by the bus, California Highway Patrol Border Division Chief Jim Abele told reporters at a press conference.

The bus, which was on Interstate 10 returning to Los Angeles from the Red Earth Casino in Thermal, California, crashed into a tractor-trailer, leaving the front of the bus demolished, Abele said Sunday. Officials said the bus arrived at the casino Saturday night between 11 p.m. and 11:30 p.m. and left Sunday at 4 a.m. The bus did not have seat belts.

It was also revealed today that the bus driver may have been inside the casino during that time.

The investigation is in a very early stage, officials said today, adding that it's too soon to say what caused the accident.

"The speed of the bus was so significant that when it hit the back of the big rig, the trailer itself entered about 15 feet into the bus," Abele said Sunday.

The National Transportation Safety Board, which is investigating the crash, asks that witnesses call the California Highway Patrol.

The bus was inspected three times since 2014, and no mechanical issues were found, Abele said Sunday. Federal Motor Carrier Safety Administration records show it had no crashes in the two years before Oct. 22 and had a satisfactory safety rating, according to The Associated Press.

ABC News' Dean Schabner contributed to this report.