SHELL - Ranger Jason Caffey was on routine patrol, miles from anywhere, when he saw it: A rockshelter that archaeologists have been excavating for a decade had been looted, scooped out in the middle with thousands of stone artifacts and bone fragments cast aside.

But since Caffey's discovery in June in rugged, remote northern Wyoming, no arrests have been made and leads have been limited. The incident points out the difficulty that a limited number of federal officials in the field face in protecting remote archaeological sites containing valuable artifacts.

Researchers now are hoping to salvage what they can at BA Cave rockshelter, a shallow hole on a mountain they believed could help provide clues about the environment and human dwellers in the region thousands of years ago.

"It's going to be a significant impact on ongoing research," said Mike Bies, an archaeologist in the BLM's Worland field office. "They moved as much dirt in one event as we moved in 10 years."

Strangely, small-scale looting in the early 1990s provided an impetus for work at the site and a research agreement between the BLM and University of Wyoming. Artifacts suggest occupation back about 7,000 years, and perhaps earlier, Bies said. That is one of the things researchers have sought to establish.

Deliberate excavations so far have yielded a wealth of artifacts and environmental data going back about 3,700 years; details have been carefully mapped and recorded, said Marcel Kornfeld, an associate professor of anthropology at UW.

Archaeologists, often working in soft dirt with paintbrushes or bamboo picks, have recovered items such as tools, bone and stone artifacts, and plant remains and pollen, which officials say help provide clues to climate and environment. Layers of cultural evidence have also been revealed, officials said.

The work originally began at the edges of the previous looting. Archaeologists hadn't touched the portion gouged out in the latest incident, which Bies said claimed half of the shelter's interior and cut through deposits 3,000 years or older.

Left behind by looters were thousands of pieces, many seemingly sorted, Bies said, that included bone pieces, stone artifacts and parts of projectile points.

"It looks like they sorted the bone and chips, like they knew they wanted something better," he said.

Just what the looters wanted or actually got isn't clear, and, officials concede, it may never be. The BLM has calculated damages at nearly $7 million.

David Tarler, an attorney and consultant on protection issues, said looting on federal lands is a problem that often goes unnoticed. "Looting isn't like robbing a bank," he said.

The BLM has just six law enforcement rangers in Wyoming, spokeswoman Cindy Wertz said, and relies on employees and the public to report problems on the roughly 18 million acres the agency oversees.

"We really rely on education, getting the word out that things they find on public land are protected," Wertz said.

But, she acknowledged, publicizing certain areas - such as the Black Mountain Archaeological District that includes the BA Cave and similar formations and is listed on the National Register of Historic Places - can be a double-edge sword.

Tim Nowak, archaeologist in the BLM's state office, said caves and rockshelters are popular targets for vandals and looters, despite their often remote or dangerous locations.

"The type of people that go after those types of resources are really interested in getting up there," he said. "They know that's where the cultural resources are untouched. It's not something the everyday vandal would do."

The rockshelter, while remote, is accessible by horse, four-wheeler or pickup, officials said, though it can be harrowing this time of year.

Even if an artifact was for sale at an online auction or at a special show, it would be very difficult - if not impossible - to know for sure where it came from, he said. Complicating matters further in this case is that officials don't even know what was taken.

The BLM is offering a $20,000 reward for information leading to the criminals' arrest and conviction. Law enforcement ranger Caffey, who believes the looting happened within weeks of his discovery, hopes the money will help provide a break in the case.

The University of Wyoming's Kornfeld likened the looting to blowing up a room in a house.

"If the bedroom from the house is gone and you're trying to evaluate what it looked like, you're not going to be able to do that," said Kornfeld, who, like Bies, believes information can still be gleaned from the site. "There are other rockshelters in the area and the project involves looking at others. But archaeology sites tend to be relatively unique and linking them together helps our understanding of prehistory."