An explosion that blew out the windows of a burning Bolt bus on the Massachusetts Turnpike was caused by a buildup of hot gases inside the vehicle, according to investigators.

Massachusetts State Police spokesman David Procopio said the investigation into the fire determined that no passengers were carrying any hazardous materials that could have caused the explosion. The fire is believed to have begun in the engine.

The Bolt bus burst into flames around 4:45 p.m. Monday on the eastbound side of the Mass Pike, east of the Weston toll booth. Smoke and flames poured from the vehicle as it sat on the side of the highway before exploding.

Windows launched into the road in the middle of rush hour traffic before firefighters were eventually able to put out the flames.

"A car that was in front of us, a man gets out and starts screaming, 'You're on fire, you're on fire.' So we're all on the bus and starting to panic," said passenger Madeline Halimi.

All 46 passengers and the driver got off the bus in time. The bus was traveling from New York to Boston.

"Being on the back of the bus, during the situation I would say I was a little bit more worried than say someone in the first row. So having never been in a life threatening experience it kind of seemed like that. I was panicking a little and there was definitely a little worry," said passenger Jake Freeman.

"I tried to make it to South Station but it was impossible," said the driver, Nora Pacheco, who said she stopped in Milldale, Connecticut, because the bus was overheating. After a mechanic worked for two hours, it continued on its way.

"But there was still this noise in the bus, tick, tick, tick. And she said don't worry about it, and kept going," said passenger Grace Metri.

"People are concerned. Another half hour passes. It's still beeping. We start to smell the smoke or gas or whatever it was," Halimi said.

There were flames in the back of the bus, but before the explosion, passengers grabbed their bags and waited on the side of the road.

Two other buses took them to the rest of the way to South Station.

"I use Bolt bus all the time. Don't know if I'm going to use it anymore though," Halimi added.

Each passenger that necn spoke with said they had been satisfied with Bolt bus, and had no negative experiences until today.

The state police investigation is now complete, but because of the interstate travel route, the Federal Motor Carrier Safety Administration will investigate whether the bus was in violation of any federal motor carrier laws.