CODY, Wyo. (AP) — With avalanche dangers looming, officials said Wednesday it could be months before crews can reach a single-engine plane that crashed on a mountain in northwest Wyoming, presumably killing two elderly brothers on board.

Robert L. Zimmerman, 84, of Huntsville, Alabama, and Ward H. Zimmerman, 86, of Seattle, are believed dead based on aerial views of the crash site on a rugged, snow-covered mountain just outside Yellowstone National Park.

The danger of an avalanche makes it too risky to put rescuers on the ground, the Park County Sheriff’s Office said. The plane crashed at an elevation of about 9,900 feet on a 60-degree slope with a large cornice looming above the wreckage.

No effort to recover the bodies and the plane can be made until the avalanche risk ends and the deep snow melts enough, Sheriff’s Office spokesman Lance Mathess said.

No one can say now when the conditions will improve, he said.

“It could be two weeks or it could be two months,” Mathess said. “The snow cornice could fall today or it could hang on for months. The mountains tend to make their own weather, and we could still get significant snowfall up there.”

The 1963 Mooney M20C was last seen May 6 departing Yellowstone Regional Airport in Cody. The plane with the Zimmermans aboard was reported missing Saturday and was found Monday on Howell Mountain, which peaks at 10,964 feet.

An attempt to use a Wyoming Army National Guard helicopter to drop a crew member down to the plane to determine the condition of the two men was abandoned after search-and-rescue members saw the crash site and the avalanche danger.

The brothers were headed from Cody to Twin Falls, Idaho, and then to Seattle. They had planned to fly over Yellowstone after taking off from the Cody airport.