CANBERRA, Australia (AP) — The search for an Australian army helicopter that crashed at sea killing four people during a military exercise with the United States last week found human remains but not the black box crucial to explaining the tragedy, an officer said on Thursday.

Retrieving the bodies of the four air crew and the black box flight data recorder have been the main priorities since an MRH-90 Taipan helicopter crashed on July 28 during a nighttime operation in the Whitsunday Islands off the northeast Australian coast.

An underwater drone spotted the human remains and part of the cockpit at a depth of 40 meters (131 feet) on Wednesday, said Army Lt.-Gen. Greg Bilton, who is coordinating the operation.

“The debris field is consistent with a catastrophic, high impact” with the ocean surface, Bilton told reporters.

The Australian navy would soon deploy specialized equipment to retrieve the wreckage and remains, he said.

The search and recovery operation, which has involved the U.S. and Canadian militaries, has been hampered by bad weather and strong currents.

Searchers were determined to recover the black box, which contains flight data and cockpit voice recordings.

“It’s a difficult task but we’ll do our absolute best to find it and, as you know, the black box is critical to helping us to understand what’s actually taken place,” Bilton said.

The crashed Taipan had been taking part in Talisman Sabre, a biennial U.S.-Australian military exercise that is largely based in Queensland state. This year’s exercise involves 13 nations and more than 30,000 military personnel.

A French Airbus helicopter had been flying with three aircraft and “communications were normal” before the crash, Bilton said.

A rescue operation began immediately but officials said on Monday there was no longer any chance of finding survivors.

Australia’s fleet of more than 40 Taipans has been grounded since the crash and there are doubts any will fly again.

The government announced in January plans to replace them with 40 U.S. Black Hawk helicopters. The Taipans’ retirement date of December 2024 would be 13 years earlier than Australia had initially planned.

Since that announcement, the fleet was grounded in March after a Taipan ditched off the New South Wales state coast near the naval base at Jervis Bay during a nighttime counterterrorism training exercise. All 10 passengers and crew members were rescued.