U.S. authorities say they will not try to force four men accused of walking onto a sensitive hot spring at Yellowstone National Park to return from their homes in Canada for prosecution.

The U.S. Attorney's Office says the men are charged with misdemeanors that don't meet the legal standard to force their return to face prosecution.

The men were traveling in a recreational vehicle with British Columbia license plates and are believed to be back in Canada.

Powell says he hopes the four will voluntarily return so the matter can be resolved in court.

If they don't, the four can either be arrested if they attempt to cross the U.S. border or denied entry.

The men are accused of leaving an established boardwalk and stepping onto a geothermal feature on May 14.