BARSTOW, California (AP) -- A $1 million race across a southern California desert by driverless robots ended Saturday after all 15 entries either broke down or withdrew, a race official said.

The race ended after the final four competitors were disabled, said Col. Jose Negron, race program manager. Competitors suffered a variety of problems that included stuck brakes, broken axles, rollovers and malfunctioning satellite navigation equipment.

One six-wheeled robot built by a Louisiana team was disqualified after it became entangled in barbed wire.

"It's a tough challenge -- it's a grand challenge -- you can always bet that it's not doable. But if you don't push the limits, you can't learn," said Ensco Inc. engineer Venkatesh Vasudevan, shortly after his company's entry rolled onto its side several hundred yards from the starting gate.

The Pentagon's research and development agency planned to award $1 million to the first team whose microcircuit-and-sensor-studded vehicle could cover the roughly 150-mile course in less than 10 hours.

The Defense Advanced Research Projects Agency was sponsoring the Grand Challenge to foster development of autonomous vehicles that could be used in combat. Defense officials foresee using the driverless, remote control-free robots to ferry supplies in war zones.

One competitor said the goal wasn't necessarily to complete the course.

"From my opinion, it's always been a question of how far you can get," said Palos Verdes High School sophomore Kevin Webb, 16. His school's entry, a modified Acura SUV, hit a barrier shortly after crossing the starting line.

Of the 21 teams that attempted to qualify over four days of trials earlier this week, just seven completed a flat, 1.36-mile (2.2-kilometer) obstacle course at the California Speedway in Fontana, east of Los Angeles.

Race organizers deployed eight to 10 tow trucks along the route in anticipation of breakdowns.

The first of the 15 entries out of the gate was Carnegie Mellon University's converted Humvee, which took off at a fast clip but stalled after half an hour.

"It is out of the race," said Carnegie Mellon senior Nick Miller, 22, a member of the team that had been the early favorite.

The defense agency spent $13 million on the race. It estimates competitors laid out four to five times that amount developing their entries, which rely on global positioning satellites as well as a variety of sensors, lasers, radar and cameras to orient themselves and detect and avoid obstacles.

Carnegie Mellon President Jared Cohon said his school's vehicle cost approximately $3 million, which was contributed by dozens of corporate sponsors.

With the on- and off-road race halted Saturday, the agency will host another contest, probably in 2006.