CITRA, Fla. -- A 13-year-old student was killed and eight others injured when a school bus crashed and caught fire, trapping the injured children in their seats.

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IMAGES: School Bus Ignites

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Officials said the bus carrying 21 students from North Marion Middle School and North Marion High School was struck by a big rig Tuesday afternoon on U.S. Highway 301 at 155th Street near Citra.

As flames began to overwhelm the bus, four motorists stopped and began pulling the trapped students out of the flames.

"They were heroes," Marion County School Superindent Jim Yancey said at the crash scene. "(The motorists) started pulling people out of the bus, there were four of them, and they kept trying to go back and get more and more kids and they said finally it just got too hot and they couldn't pull anymore of kids out. They know there was a kid trapped in the back and they just couldn't get him or her out -- I don't know which it was. They just couldn't get him out and they were showing more grief and remorse for not being able to do more. There were four gentlemen who just jumped into the inferno."

Local 6 reported that Frances Shee, 13, burned to death inside the bus.

Six other students pulled from the bus and two adults were transported to a nearby hospital. Four remained in the hospital late Tuesday.

"It is just a really sad day for Marion County schools," Yancy said. "Those 18 kids who got out of there alive are very fortunate to get out of there alive. This is a tragedy but it is also a miracle for those kids who got out of there."

Yancy said he can't believe anyone survived the crash.

"It is an unbelievable scene of destruction and devastation and we are lucky one person got out of their alive," Yancy said.

Witnesses said the students were shocked after the crash.

Troopers said the big rig was driven by Reinaldo Gonzalez, 30, from Orlando. Gonzalez was transported to the hospital with serious injuries.

The bus was apparently sitting at a stop sign with its flashers engaged before the collision.

The big rig hit the bus and pushed it hundreds of feet before both vehicles caught fire, investigators said.

The cause of the crash is under investigation.

U.S. Highway 301 was closed in the area but reopened to traffic late Tuesday.

Watch Local 6 News for more on this story.