LAKE BUTLER, Fla. -- Seven children were killed Wednesday afternoon when a Union County, Fla., school bus, a tractor-trailer and a car crashed.

According to the Florida Highway Patrol, the crash occurred just after 3:30 p.m. on state Road 121, four miles south of Lake Butler.

A police officer described the scene as "a mangled, charred mess."

Seven children in the car, ranging in age from 21 months to 15 years, were killed. Police said the truck hit the car from behind and pushed it into the bus, causing the car to burst into flames. The dead were all related and police said there was no adult in the car.

Investigators said evidence from the scene shows the truck did not brake before hitting the car. The bus ended up 200 feet from where the car struck it.

At least three of the nine children on the bus were seriously injured. The drivers of the bus and the truck were also taken to a hospital.

The National Transportation Safety Board is sending a team of investigators.

State Road 121 is a rural, two-lane road, and weather was not a factor in the crash, Jacksonville, Fla., television station WJXT reported.