When three suspicious men were stopped on federal land in remote northwestern Nebraska in 2003, it didn't take the U.S. Forest Service long to figure out what they were doing.

They were poaching fossils -- a practice the Forest Service says has become rampant in recent years at Oglala National Grassland.

The men had dug an 18-by-10-foot hole more than 2 feet deep, leaving the fossilized bones of a prehistoric rhinoceros exposed. Plaster used to take casts of the bones and excavating tools also were found. The men were arrested and convicted in federal court, but Forest Service paleontologist Barbara Beasley said most fossil poachers are never caught.

Fossil poaching has not exactly been a priority for federal officials. Only one federal law enforcement officer patrols 1.1 million acres of federal grasslands in Nebraska and South Dakota. But that is changing.

Signs of poaching show up nearly every week, Beasley said. Exposed holes and excavation tools are routinely found on federally protected grasslands. Of more than 162 grassland areas identified in the 1990s as holding fossils, about 30 percent showed evidence of poaching, she said.

Although the problem is prevalent in fossil-rich areas, from Colorado to Montana, the Forest Service said it is particularly bad in Nebraska because of the lack of natural barriers such as mountains or thick brush that may hinder access.

"We have researchers and academic scientists who find our permitting process difficult and just decide to go around it," Beasley said. "But a lot of them just want to sell fossils."

Dinosaur fossils also turn up by the hundreds at shows, in catalogues and on Internet auction sites. Fossilized skulls of prehistoric animals can sell for thousands of dollars on eBay. In June, a saber-toothed cat skull sold for $32,312 at a Bonhams & Butterfields natural history auction.

Beasley and others who conduct field work on federal lands are being trained to be forest protection officers. That gives them the authority to investigate criminal cases, although not to carry firearms. "There's been more attention paid to poaching . . . a lot of it because of the higher profile of fossils as the black-market prices climb," Beasley said. "Our plan is to deter unauthorized collecting."

Barbara Beasley of the Forest Service points out exposed dinosaur fossils in the Oglala National Grassland in Nebraska.