BOZEMAN, Mont. -- A national park advocacy group is calling on the National Park Service to do a better job of keeping snowmobiles from trespassing in Yellowstone National Park.

"They're simply not patrolling the boundary and backcountry as much as this kind of trespass should warrant," Bill Wade, chairman of the Coalition of National Park Service Retirees, said Thursday.

Aerial photographs taken in February showing telltale signs of snowmobiles crisscrossing the park's western boundary. The photos were taken near Smokejumper Springs and South Riverside Cabin by retired Yellowstone ranger Bob Peterson, who lives in Jackson, Wyo.

In a letter accompanying photos sent to Yellowstone Superintendent Suzanne Lewis, Peterson called this year's incursions, "the most extensive noted in the last couple of years."

"Certainly (snowmobile trespassing) is not going away, and I do not believe that it will become less of an issue in the future," he wrote.

Park spokesman Al Nash said he had not seen the letter or photographs, but was aware of the recurring violations. "It has occurred, I know it continues to occur," he said Thursday.

But he said enforcing park-boundary rules in such a large area is a challenge.

Yellowstone Park has 50 permanent law-enforcement rangers and they issued 2,600 tickets in 2007, Nash said. Of those, 12 were "snow-machine violations," which covers a broad spectrum, from trespassing to speeding on designated trails.

The incursions could be accidental, said Brad Grein, co-executive director of Citizens for Balanced Use, a multiple-use advocacy group that works to keep public land open to motorized vehicles.

"As far as the Park Service is concerned, they do mark their boundaries," he said. "But at times there are areas where it is feasible that a person would cross over into the national park accidentally."

Grein said CBU works with the U.S. Forest Service -- which manages much of the public land surrounding the park and most of the land snowmobile riders use -- to make sure boundaries are clearly marked.

Wade suggested the signs of incursions shown in Peterson's photographs suggest snowmobiles should be further restricted in Yellowstone.

"It's another example of how snowmobiles are impacting the resource of the park," he said.