Jordan, Minn. ? A 17-year-old boy was behind the wheel of a semi pulling a box trailer converted into a recreational vehicle when the 57,000-pound rig crashed through a guardrail and into a Kansas ravine, killing five of the 18 people on board.

Adam Kerber’s driver’s license wouldn’t have allowed him to drive a commercial vehicle like that because of its weight and because it was carrying more than 15 people. But neither of the restrictions applied because of a loophole in Minnesota state law regarding private RVs.

The 13 injured in the crash included Kerber, who was still in critical condition Monday. All those injured or killed were friends or members of the Kerber family.

The crash happened about 9 a.m. Sunday as the family returned from an annual motocross vacation in Texas. Their Freightliner cab and Haulmark trailer broke through a guardrail on Interstate 35 in Kansas and plunged into a ravine. Kerber and another teen were the only people wearing seat belts.

A neighbor familiar with the trailer said he didn’t believe it even had seat belts, which aren’t required in Minnesota other than in a vehicle’s front seat.

John Marks, of Jordan, told The Associated Press that several of the Kerber children were motocross racers, who referred to the family rig as a “toterhome” because they used it to tote their motorcycles and other equipment to events.

Marks, who had been inside the mobile home, said the box trailer was divided into two sections, with furnished living quarters in the forward end with a refrigerator, store, TV, toilet, and a separate bedroom. Motorcycles and equipment was kept in back. As far as he knew, there were no seat belts in the trailer.

Many companies sell or modify trailers for use as recreational vehicles. They are especially popular with motor sports enthusiasts and horse breeders.

Tom Meyer, who runs Tom’s Custom Coach and Trailers in Independence, Mo., said most manufacturers or modifiers advise people not to ride in them without seat belts. But customers “are pretty lax about that,” he said.

“They think they’re back in a big motor home, they’re pretty safe,” he said.

The Kerber family was headed by Pauline Kerber, 46, who was in stable condition Monday at Overland Park Regional Medical Center in Kansas.

The Kansas Highway Patrol identified the dead as Tom Kerber, 25, of New Prague, Minn., and Jessica Kerber, 10, Joy Kerber, 14, and James Kerber, 12, of Jordan, Minn. Tom Kerber’s wife, Melissa Kerber, 24, also died.

Those injured ranged in age from 2 to 30 and were taken to various hospitals.