Five children on their way to Walt Disney World and two adults were killed in a fiery multivehicle crash in northern Florida on Thursday, the authorities said. At least nine others were injured.

The crash occurred on Interstate 75 near Gainesville, Fla., at about 3:40 p.m., when a tractor-trailer and a car collided in a northbound lane, according to the authorities. The vehicles careered through a guardrail into southbound traffic, striking a van carrying the children and another tractor-trailer.

“Once the semis struck, they both caught on fire, as well as the passenger car,” Lt. Patrick Riordan, of the Florida Highway Patrol, said on Friday morning at a news conference.

It is unclear what caused the vehicles to collide. The crash is under investigation.

About 50 gallons of diesel fuel spilled onto the highway, intensifying the flames and releasing plumes of black smoke. A fifth vehicle struck either some of the injured or some debris, he said.