Six teenage girls on a high school lunch break were killed when their small car with only four seats collided with a large truck hauling rocks, the Oklahoma Highway Patrol said Wednesday.

The crash occurred shortly after noon Tuesday in Tishomingo, a rural city of about 3,000 located about 100 miles (160 kilometers) southeast of Oklahoma City, the patrol said. Those killed included the 16-year-old driver, three 15-year-olds, and two 17-year-old passengers, according to the patrol.

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While what led to the crash is unknown, it highlighted concerns of teenagers carrying other young passengers.

“Just adding a single passenger under age 21 increases the risk of crashing by 44%” when the driver is a teen," said William Van Tassel with AAA’s national office.

The crash report, released Wednesday morning, said the circumstances of the wreck remained under investigation. But Highway Patrol Trooper Shelby Humphrey said Tuesday night that the girls’ car was making a right turn when it collided with the truck, KXII-TV reported.

"One of the main concerns and risks of having multiple teenagers in a car is the distractions that come with that,” Van Tassel said.

“If one of the passengers is over 35 (the risk) goes down by 62%. That implies teens can drive safely when there’s an adult in the car," Van Tassel told The Associated Press.

Only the 16-year-old driver and front-seat passenger were wearing seat belts when the 2015 Chevrolet Spark collided with the truck, according to the Highway Patrol.

“The unbelted people put everyone at risk," Van Tessel said. "In a crash, the unbuckled people fly around all over the place,” injuring others inside the vehicle.

Oklahoma is the only state where passengers who are older than 7 years old and in the back seat of a car do not have to wear a seat belt, said Leslie Gamble, the manger of public and government relations for AAA-Oklahoma.

“A 41-member coalition of traffic safety advocates has pushed for a bill to be passed by our state legislators for the past three years without success,” Gamble said

The crash occurred about a mile (1.6 kilometers) from Tishomingo High School.

Students in the district of about 850 students were in class Wednesday, Tishomingo Public School Superintendent Bobby Waitman said.

“Academics are secondary, frankly, at this point to the students knowing that they belong, that they have a safe place,” Waitman said.

“You'll never fully understand, I don't think we'll ever fully understand a loss like this," Waitman added.

The girls' names weren't released because they are juveniles.

The Highway Patrol identified the driver of the truck as Valendon Burton, 51, of Burneyville, Oklahoma. The report said Burton was not injured.

The National Transportation Safety Board was sending a team, according to NTSB spokesman Peter Knudson.

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Waitman said funerals for the students were not yet scheduled and that the district would work with their families to potentially schedule a memorial service on campus.

The crash happened one week after nine people were killed — including six members of a New Mexico college’s golf team and their coach — died in a crash in West Texas. In that crash, the NTSB determined that a 13-year-old boy was behind the wheel of a truck when it blew a tire and struck the van carrying University of the Southwest students.