TWENTYNINE PALMS, Calif. (AP) - Hundreds of Marines were conducting a combat training mission in the Mojave Desert when an air patrol spotted something kicking up dust: A civilian pickup truck speeding across the barren landscape.

Behind the wheel was a suspected scrap metal thief who had been combing the Marine Corps Air Ground Combat Center for spent brass shell casings. His intrusion onto the base was the 12th time in six months that scavengers had inadvertently halted combat exercises.

Bombing ranges have become prime hunting grounds for so-called "scrappers," who are motivated by soaring commodity prices to take greater risks in their quest for brass, copper and aluminum. The scavenging causes headaches for the military, which cannot patrol every inch of the remote bases where spent ammunition, shrapnel and unexploded ordnance are easy to find.

"This is not just some petty crime. This is dangerous business," said Andy Chatelin, director of range management at Twentynine Palms, which at 932 square miles is the world's largest Marine Corps base.

Illegal scavenging of military munitions has long been an issue at military bases. But as metal prices have climbed in the past two years, scavengers have become more numerous, more audacious and more sophisticated.

After he was spotted by troops last December, the pickup truck driver barreled directly at a Marine, who fired five shots at the vehicle. The driver swerved, flipped over and spilled hundreds of dollars in collected metal. He was taken by helicopter to a hospital and later charged with attempted murder.

The military loses hundreds of thousands of dollars every time it is forced to halt training. And when scrappers make off with unexploded ordnance, the public is at risk.

The Pentagon estimates up to 10 percent of all ordnance such as bombs, missiles and grenades fails to explode on impact. Some of it is left behind in training areas.

In May 2007, two suspected scrappers removed a Vietnam-era missile from the Twentynine Palms base. It later exploded in their Barstow home, killing both men and destroying the apartment. Earlier this year, two workers were injured at a Raleigh, N.C., recycling plant when ordnance suspected of coming from nearby Fort Bragg exploded.

"The expense we have to go through, not just to guard against the loss of training time, but the chance of this hazardous material getting out into the public, is enormous," said Ronald Pearce, who oversees a training range in Yuma, Ariz., where the Marines and Navy practice aerial assaults. "You just can't look the other way and condone it."

No one knows how much scrap metal lies discarded on U.S. military bases because there are no records of the tonnage of exploded and unexploded ordnance. The number of illegal scavengers is also unclear because the military can only confirm a theft when there is an arrest.

After meeting with the Defense Department last month, the Institute of Scrap Recycling urged its members to stop accepting military scrap without knowing the source of the material. It also recommended the military create a system to account for the material it uses.

The Pentagon said it's impossible to calculate the cost in interrupted training - including lost man-hours and wasted fuel - but they have begun tracking lost training time, which can climb into the hundreds of thousands of dollars.

At the Marine Corps Air Station in Yuma, officials estimated they lost nine hours of aerial training between January and March.

To catch thieves, bases are combining technology with foot patrols and relying on help from sheriff's deputies.

The Twentynine Palms base is using cameras to conduct video surveillance of base borders. It also has assigned Marines from its Special Reaction Team, similar to a SWAT team, to work primarily on nabbing scrappers and trespassers.

But they are often up against a savvy enemy that uses high-tech communications and GPS systems, and often works in teams.

During a recent patrol at the base, Marines hunted for scrappers in gullies, desert washes and mountain crevices where some thieves had previously hid from helicopters under camouflage netting.

Last year, Marines found an abandoned car in the desert and a dead man nearby, plus a second man who was on the brink of death from dehydration. The pair were believed to have been prowling for scrap metal. Similar deaths were reported in Yuma.

The military said most scrappers arrested in the past several years appeared to be either illegal immigrants or drug users looking for easy money. If convicted on federal charges ranging from trespassing to theft, they face up to 20 years in prison.

Because the Twentynine Palms base is so vast, officials cannot erect and maintain fences. Instead, they have posted signs warning against trespassing, only to see those signs stolen for the metal.

"We've seen all types," Sgt. Timothy Warren said as he scanned the mountains with binoculars, looking for scavengers. "We've even arrested one guy, sent him to jail and then arrested him again a few days before he's even gone to court."