A rescue team on an air force helicopter has spotted what appears to be a small aircraft window in a forested area on the slope of Mount Bulusaraung.

Indonesian rescuers have recovered wreckage from a missing plane that is believed to have crashed with 11 people on board while approaching a mountainous region on Sulawesi island during cloudy conditions.

The discovery on Sunday comes after the small plane – on its way from Yogyakarta on Indonesia’s main island of Java to Makassar, the capital city of South Sulawesi province – vanished from radar on Saturday.

A rescue team on an air force helicopter on Sunday morning spotted what appeared to be a small aircraft window in a forested area on the slope of Mount Bulusaraung, said Muhammad Arif Anwar, who heads Makassar’s search and rescue office.

Rescuers on the ground then retrieved larger debris consistent with the main fuselage and tail scattered on a steep northern slope, Anwar told a news conference.

“The discovery of the aircraft’s main sections significantly narrows the search zone and offers a crucial clue for tightening the search area,” Anwar said. “Our joint search and rescue teams are now focusing on searching for the victims, especially those who might still be alive.”

The plane, a turboprop ATR 42-500, was operated by Indonesia Air Transport and was last tracked in the Leang-Leang area of Maros, a mountainous district of South Sulawesi province.

It was carrying eight crew members and three passengers from the Marine Affairs and Fisheries Ministry who were on board as part of an airborne maritime surveillance mission.

Ground and air rescue teams continued moving towards the wreckage site on Sunday, despite strong winds, heavy fog and steep, rugged terrain that had slowed the search, said Major-General Bangun Nawoko, South Sulawesi’s Hasanuddin military commander.

Photos and videos released by the National Search and Rescue Agency on Sunday showed rescuers were trekking along a steep, narrow mountain ridgeline blanketed in thick fog to reach scattered wreckage.

Indonesia relies heavily on air transport and ferries to connect its more than 17,000 islands. The Southeast Asian country has been plagued by transport accidents in recent years, from plane and bus crashes to ferry sinkings.