A Greyhound passenger bus crashed into three tractor-trailers parked along a highway rest area exit early Wednesday in southern Illinois, killing three people and injuring 14 others, some seriously, state police said.

The St. Louis-bound bus was traveling westbound along Interstate 70 in Madison County when it crashed into the three semis just before 2 a.m., Illinois State Police said, citing an initial investigation.

Four people were taken to the hospital by helicopter and at least 10 others were taken by ambulance, state police said in a news release. Police did not immediately release details about those who were injured and killed.

No one in the three trucks was hurt in the crash near the city of Highland, about 25 miles (40 kilometers) east of St. Louis, police said.

State Police spokesperson Melaney Arnold said those killed and injured were all on the bus.

Amalgamated Transit Union, which represents Greyhound drivers, said in a Facebook post that the driver was in serious condition at a hospital.

The crash closed westbound traffic on I-70.

A team from the National Transportation Safety Board arrived at the scene Wednesday. U.S. Rep. Mike Bost, an Illinois Republican, said an NTSB official told him the bus was equipped with monitoring cameras “so they’ll be able to do a full check to see how the accident occurred.”

Photos and video from the scene showed the side of the bus peeled open and its roof crumpled. A second tractor-trailer appears to have made contact with the right rear of the bus while a third tractor-trailer appears to have crashed into the rear of that second semi.

Passenger Edward Alexander of Pine Bluff, Arkansas, told the St. Louis Post-Dispatch he helped a pregnant woman get off the bus and was searching for his phone when he “realized smoke was coming in the bus. I was like, ‘forget that phone,’ and went on and jumped out the window.”

Edwin Brown, 22, of Friars Point, Mississippi, told the Post-Dispatch that he felt the bus shake as it passed over rumble strips before the side of the vehicle “opened up like a can opener.” The driver was in and out of consciousness as Brown turned the ignition off with the help of a trucker, he said.

Greyhound spokesperson Mike Ogulnick said in an email that the bus was traveling from Indianapolis to St. Louis with a scheduled arrival of 2:20 a.m. About 30 people were on board.

“Our primary concern is ensuring we care for our passengers and driver at this time,” Ogulnick said. “We are working closely with local authorities, and a relief bus is on the way for passengers.”

It is illegal in Illinois for trucks to park on exit ramps. But trucking industry experts say semis often stop there for the night because overnight parking is hard to find at rest areas and other places such as truck stops.

“And that’s not only dangerous for them but it’s dangerous for the motoring public because they do need their rest and they deserve their rest,” Lewis Pugh of the Owner-Operator Independent Drivers Association said at a May hearing before a House Committee on Transportation and Infrastructure subcommittee.

Tom Chapman, an NTSB board member, declined to discuss specifics of the crash during a briefing Wednesday, including the reports that the rigs were stopped along the ramp.

“Rest area safety is one of the issues that will be a part of this investigation,” he said. “Again, we don’t know enough to be able to say with certainty that that’s what occurred, but that’s certainly one of the issues that we’ll be looking at as part of our investigation.”

The findings of the inquiry could lead to recommendations designed to “help ensure that similar tragedies not occur in the future,” he added. Another briefing was planned for Thursday.

Last month the National Highway Traffic Safety Administration announced that it will require trucks and buses to include automatic emergency braking equipment within five years. AEBs use forward-facing cameras and sensor technologies to detect when a crash is imminent.

The system automatically applies the brakes if the driver has not done so, or, if needed, applies additional braking force to supplement the driver’s actions. The proposed standard would require the technology to work at speeds ranging from 6 to 50 mph (10 to 80 kph).