Indonesian authorities will download on Monday the contents of a cockpit voice recorder (CVR) from a Lion Air jet that crashed more than two months ago, killing all 189 people on board, after it was retrieved from the sea near Jakarta.

The crash was the world's first of a Boeing Co 737 MAX jet and the deadliest of 2018, and the recovery of the aircraft's second black box earlier on Monday may provide an account of the last actions of the doomed jet's pilots.

"We have our own laboratory and personnel team to do it," Haryo Satmiko, deputy chief of the transportation safety committee, told Reuters.

Satmiko said it had in the past taken up to three months to download, analyze and transcribe the contents of recorders.

Contact with flight JT610 was lost 13 minutes after it took off on Oct. 29 from the capital, Jakarta, heading north to the tin-mining town of Pangkal Pinang.

A preliminary report by Indonesia's transport safety commission focused on airline maintenance and training, as well as the response of a Boeing anti-stall system and a recently replaced sensor, but did not give a cause for the crash.

A group of relatives of victims urged the transportation safety committee to reveal "everything that was recorded" and to work independently.

Navy officer Lieutenant Colonel Agung Nugroho told Reuters a weak signal from the recorder was detected several days ago and it was found buried deep in soft mud on the sea floor in water about 30 metres deep.

"We don't know what damage there is but it has obvious scratches on it," Nugroho said.

Pictures supplied by an official from the transportation agency showed bright orange paint on the CVR memory unit chipped, but no major dents.

Nugroho said human remains had been found near where the CVR was discovered, about 50 metres from where the crashed jet's other black box, the flight data recorder (FDR), was found three days after the crash.

Investigators brought in a navy ship last week after a 10-day, 38 billion rupiah ($2.70 million), effort funded by Lion Air failed to find the recorder. Bureaucratic wrangling and funding problems had hampered the initial search.

The L3 Technologies Inc CVR was designed to send acoustic pings for 90 days after a crash in water, according to an online brochure from the manufacturer.

That would mean that after Jan. 27, investigators could have faced a far bigger problem in finding the CVR buried along with much of the wreckage deep in mud on the sea floor..

Boeing did not respond immediately to a request for comment.

Since the crash, Lion Air has faced scrutiny over its maintenance and training standards, and relatives of victims have filed at least three lawsuits against Boeing.