Four people were killed and another person is missing and presumed dead following a crash involving a sightseeing plane in Alaska‘s Denali National Park over the weekend.

According to Fox News, the aircraft was carrying a group of Polish tourists when it crashed on a mountain ridge near the summit of Thunder Mountain about 14 miles southwest of Denali—North America‘s tallest mountain—at around 6 p.m. local time Saturday.

The pilot sent out a distress call and reported by satellite phone that passengers suffered injuries but was unable to provide details before the connection failed. Low-lying clouds and rainy conditions prevented search and rescue crews from spotting the crash site until Monday.

The National Park Service (NPS) described the terrain as “extremely steep and a mix of near-vertical rock, ice and snow.”

A park service ranger who was able to reach the site by short-haul line below a helicopter found the four bodies inside the snow-filled aircraft. There were no footprints or other disturbances in the snow indicating that anyone had made it out of the plane.

The aircraft was operated by K2 Aviation and was touring the Kahiltna Glacier when it crashed. The glacier is the longest of the Alaska Range and the starting point for climbers attempting to ascend Denali.

The NPS said a temporary flight restriction remains in place over part of the park to minimize traffic near the crash site.