On May 3, Eric Bjornstadt had just finished hiking the length of Horseshoe Canyon, a western tributary to the Green River renowned for spectacular Barrier Canyon-style pictographs believed to be more than 2,000 years old.

As he fought through a dense patch of tamarisk at the mouth of the canyon, he came face-to-face with a brilliant red-and-white rainbow painted on the canyon wall."It was beautiful," Bjornstadt said. Like Bjornstadt, those who have seen the Rainbow Pictograph now use "was" instead of "is" in describing the panel's beauty, because chiseled into the center of the rainbow is the name "H.J. Hogan" and the date "4-30-92."

The vandalism has sparked not only a manhunt for H.J. Hogan but a mystery that has sent researchers and investigators scurrying to find anything and everything they can about the rock art panel.

"It may have been vandalized three days before (Bjornstadt's visit) or it could be 100 years old," said Bureau of Land Management archaeologist Bruce Louthan. "We do have a 1903 date in an alcove nearby. The problem we've had investigating this case is that we haven't been able to find a photo of the panel (from before the April 30 date of the vandalism)."

The criminal investigation has reached a virtual dead end, and Louthan is appealing for public help in locating H.J. Hogan or even a photograph of the panel taken before April 30, 1992. "We don't want to be chasing a guy who's dead," Louthan said.

Whoever H.J. Hogan is, federal investigators and scores of outraged Utahns are determined to track him down and - if he's still alive - prosecute him for violating the Archaeological Resources Protection Act.

Private citizens have raised more than $1,500 in reward money for information leading to the arrest and conviction of the vandal. A conviction could result in two years in prison and a $20,000 fine.

The location of the vandalism is puzzling, Louthan said, because the site is extremely remote, is virtually unknown to river runners and is hidden behind a curtain of tamarisks. It is so remote that river rangers rarely patrol the area, located on BLM land just outside the Maze District of Canyonlands National Park.

"The style of writing and its hidden location, it could well be 100 years old," Louthan said. "What we really need is a photograph."

Harvard archaeologists visited the site in the early 1930s, but their descriptions make no reference to the vandalism. The site was added to the National Register of Historic Places in 1969, but again no mention is made of the vandalism.

Bjornstadt, who has made a crusade out of locating H.J. Hogan, is convinced the vandalism is recent. He has spoken to people who have visited the rock art site, but none remembers any vandalism.

Concerned citizens recently published a full-page notice in a Moab paper labeling H.J. Hogan an "arrogant vandal or ignorant fool" and requesting help in locating the person responsible.

For reasons not readily understood, white men have felt compelled to carve their names over Utah's Indian rock art. The first fur trappers in Utah did it, as did the first pioneers.

"Whether this incident is 100 years old or if it was done this year, it's a shame either way," Louthan said. "It's a shame someone would be so insensitive as to put their name across the middle of it."

Anyone with information about H.J. Hogan or who may have a photograph of the Rainbow Pictograph is encouraged to contact Louthan at 259-2154 or BLM investigator Bart Fitzgerald at 1-800-722-3998. Anyone interested in contributing to the reward fund should contact the Back of Beyond Bookstore in Moab.