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A 13-year-old was behind the wheel of a pickup truck that slammed head on into a van carrying university athletes in a collision that killed nine on Tuesday.

In a Thursday news conference, federal investigators said the truck's left tyre - which was a spare - blew out before swerving into the oncoming van.

Six university golfers, including one from Portugal and one from Mexico, and their coach died in the crash.

Both the young pickup driver and his passenger were also killed.

Two students, both from Canada, survived the collision and are in a stable condition but still in hospital.

Texas Department of Public Safety Sgt Victor Taylor told the Associated Press news agency that it would be illegal for a 13-year-old to be driving in the state. Children are allowed to begin classroom driving lessons at the age of 14 in Texas, and must be 15 before they can start driving with an instructor or licensed adult in the car.

Officials from the National Transportation Safety Board (NTSB) said in Thursday's news conference that it's still unclear how fast the vehicles were moving when they collided head-on on a two-lane highway near the New Mexico border, causing both vehicles to burst into flames.

Several of the victims in the van were not wearing seatbelts, the NTSB said, adding that one passenger was ejected from the vehicle. The information is preliminary, officials added, and the investigation into the "mass casualty event" is expected to last several more months.

The victims from the University of the Southwest - a private university in New Mexico - include golf coach Tyler James, 26; Jackson Zinn, 22; Karisa Raines, 21; Travis Garcia; 19; Laci Stone, 18, as well as Portuguese national Tiago Sousa, 18; and Mexican citizen Mauricio Sanchez, 19.

Henrich Siemens, 38. was named as the pickup truck passenger. The name of the boy driving the vehicle is not being released due to his age.

The accident occurred in Andrews County in west Texas on Tuesday night, just after 20:00 local time (01:00 GMT).

A Ford Transit passenger van was carrying members of the both men's and women's golf teams from a golf tournament, authorities said. The other vehicle involved was a Dodge 2500 pickup truck.

The Ford had been travelling northbound on the FM1788 roadway in Texas, while the Dodge pickup was travelling southbound.

The two surviving golfers are undergoing medical treatment in Lubbock, Texas.

Their families have arrived from Canada to be with them, a university official said on Thursday, adding that one is doing well enough to be able .