It sounds like the stuff of science fiction: An army of driverless vehicles crisscrossing the Mojave Desert (search), guided across the rugged landscape by an array of sophisticated software that could one day spare soldiers from the risks of battle.

On Saturday, four robots zipped across the finish line of a 132-mile race through the desert on a quest for a $2 million prize to help develop next-generation war machines. The robot pack was led by a design from Stanford University (search).

"The impossible has been achieved," said Stanford computer scientist Sebastian Thrun (search), whose teammates doused him with ice water after the checkered flag went down.

The winner of the $2 million prize was not immediately declared because 22 of the 23 contestants sprinted from the starting line at staggered times, racing against the clock rather than each other.

The last vehicle to run, a modified Ford SUV, got caught in a bush less than a mile from the start.

This year's Pentagon-sponsored race showcased a dramatic leap in robotics technology since last year's inaugural competition, which ended without a winner.

Carnegie Mellon University's (search) Humvee, dubbed Sandstorm, chugged the fastest last year despite covering only 71/2 miles. Sandstorm competed again this year and finished the course.

"I'm on top of the world," William "Red" Whittaker, a Carnegie Mellon robotics professor, said after his other entry, a red Hummer named H1ghlander, arrived at the finish line Saturday.

The sentimental favorite, a Ford Escape Hybrid (search) by students in Metarie, La., was the fourth vehicle to finish Saturday. The team lost about a week of practice and some lost their homes when Hurricane Katrina blew into the Gulf Coast.

Race officials planned to resume racing Sunday so that the sole robot left -- a mammoth, six-wheel truck -- could compete in daylight.

The vehicles were equipped with the latest sensors, lasers, cameras and radar that feed information to onboard computers, which helped the robots distinguish a dangerous boulder from a tumbleweed and decide whether a chasm was too deep to cross.

Within the first hour of the race, a half-dozen vehicles were knocked out by sensor problems. Even so, most had covered more distance than Sandstorm last year. As the day went on, more vehicles were sidelined because of problems including a flat tire.

The race was sponsored by the Pentagon's Defense Advanced Research Projects Agency (search), or DARPA. The agency will award the taxpayer-funded prize to the vehicle that completes the course the quickest.

The race is part of the Pentagon's effort to fulfill a congressional mandate to cut causalities by having a third of the military ground vehicles unmanned by 2015.

The military currently has a small fleet of autonomous ground vehicles stationed in Iraq and Afghanistan, but the machines must be remotely controlled by a soldier who usually rides in the same convoy.

The unmanned vehicles used their computer brains and sensing devices to follow a programmed route, consisting of rough, winding desert roads and dry lake beds filled with overhanging brush and other obstacles. They also had to pass through three tunnels designed to knock out their GPS signals.

About five miles from the finish line was a tricky 1.3-mile mountain pass dubbed "Beer Bottle Pass" because of its shape. The mountain ridge -- similar to mountain canyons found in Afghanistan -- was only 10 feet wide and had a 200-feet drop-off. The finishers navigated the section with no problems.

A few miles before the pass, Stanford's robot, which ran second in the race, overtook H1ghlander, which snagged the pole position. Whittaker said it appeared H1ghlander suffered a technical glitch that allowed Stanford to pass by.

To qualify, vehicles competed in a weeklong trial at the California Speedway (search) outside Los Angeles, where they had to zip through a 2.5-mile bumpy track littered with hay bales, traffic cones and junk cars. All 23 finalists completed the course at least once.

Even before Saturday's race, many teams tested their vehicles in parts of the desert under race-like conditions, including some teams that practiced on last year's course from Barstow, Calif., to Primm.

Cornell University's military light-strike vehicle traveled about nine miles when it failed to cross a bridge. Team members were trying to figure out what went wrong.

"We're at a loss," said Ephrahim Garcia, a Cornell (search) mechanical engineer. "It's a disappointment."

Each contestant was followed by a chase vehicle. To ensure safety, a judge in the chase vehicle could pause a robot during the race, stopping the 10-hour clock without penalty. The judge also could press a kill switch if the robot was headed toward danger, ending its chances of winning.