Search crews hunting for the remains of Lion Air Flight JT610 have found the data recorder, engines and body parts of some of the 189 victims. But the key to solving the mystery behind the crash is still missing: the cockpit voice recorder.

Indonesia’s rescue agency said Sunday that wreckage spotted by divers had turned out to be only the skin of the Boeing Co. jet rather than the main fuselage. Strong underwater currents in the Java Sea off Jakarta and a muddy seabed have complicated a week-long hunt that’s involved dozens of ships and hundreds of specialist personnel.

Flight JT610’s audio black box, as it’s known, may be crucial to unraveling what happened during the flight’s final moments on Oct. 29. In particular, the device may explain why the crew asked to return to base minutes into the journey, and reveal any exchanges in the cockpit before the fatal high-speed plunge into the sea.

As the 270-square-mile search for debris widened at the weekend, Indonesian authorities broadened a review of Lion Air’s operations, including the airline’s standard operating procedures and flight-crew qualifications. That followed the discovery of defects on two other Boeing 737 Max 8 planes—both operated by Lion Mentari Airlines—during checks on six aircraft of that type.

Because Flight JT610 lasted only a few minutes, the voice recorder is also likely to include at least some audio from the previous night’s trip from Denpasar, Bali to Jakarta. The aircraft, which would crash the next day, experienced problems on the flight from Bali with sensors used to calculate altitude and speed. The instruments were checked by maintenance workers overnight and the plane was cleared to fly, according to Lion Air.

With the data recorder in the hands of investigators, Indonesia’s National Transportation Safety Committee has recovered about 69 hours of flying data by the crashed jet during its last 19 trips, it said on Sunday. The agency will begin analyzing the information from Monday to find out the reasons for the crash, Nurcahyo Utomo, the head of aviation accidents investigation sub-committee, told reporters.

The inspection of plane debris indicated the aircraft didn’t explode mid-air before plunging into the Java Sea, Soerjanto Tjahjono, chief of the safety committee, said at a briefing in Jakarta on Monday. The agency expects to release its preliminary findings within a month, he said.

“The aircraft broke apart under the impact of hitting the water at high speed and it didn’t break apart mid-air,” Tjahjono said. “The engines were still running at high RPM.”

The nation’s domestic airline market has boomed in recent years to become the fifth largest in the world. Local air traffic more than tripled between 2005 and 2017 to 97 million people, according to the CAPA Center for Aviation, and is dominated by flag carrier PT Garuda Indonesia and Lion Air Group.

Carriers have struggled with safety issues, partly as a result of the pace of that expansion, as well as issues intrinsic to a region of mountainous terrain, equatorial thunderstorms and often underdeveloped aviation infrastructure.

President Joko Widodo has asked airlines to make passenger safety the highest priority, and the government had already ordered a review of Lion’s repair and maintenance unit and suspended several managers.

The transport ministry is coordinating with airport authorities, navigation operators and airlines among others to ensure airworthiness at all airports in Indonesia is well maintained, according to Transport Minister Budi Karya Sumadi.