The captain of the doomed plane that crashed at a "high rate of descent" made a chilling call for help just moments before the tragedy.
Head of the search and rescue operation Danang Priandoko told local media the pilot, Bhavya Suneja, asked to return to Jakarta's Soekarno-Hatta airport just minutes after takeoff.
Preliminary flight tracking data from Flightradar24 shows the aircraft climbed to around 5,000 feet (1,524 m) before losing, and then regaining, height, before finally falling towards the sea.
It was last recorded at 3,650 feet (1,113 m) and its speed had risen to 345 knots, according to raw data captured by the respected tracking website, which could not immediately be confirmed.
Flightradar24 said "final date received from the aircraft indicates a high rate of descent."
Edward Sirait, chief executive of Lion Air Group, told Reuters: "We cannot give any comment at this moment,"
"We are trying to collect all the information and data."
Its last recorded position was about 15 km (9 miles) north of the Indonesian coastline, according to a Google Maps reference of the last coordinates reported by Flightradar24.
The plane was airworthy, and had only been operated by Lion Air Group since August, the company said.
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Its pilot and co-pilot had together amassed 11,000 hours of flying time, Lion Air said in a statement.
Three of those on board were trainee flight attendants and one was a technician.
At least 20 employees from Indonesia's finance ministry were on board, the BBC learned.
Manufacturer Boeing is aware of the airplane accident reports and is "closely monitoring" the situation, its spokesman told Reuters.
The accident is the first to be reported that involves the widely-sold Boeing 737 MAX, an updated, more fuel-efficient version of the manufacturer's workhorse single-aisle jet. The first Boeing 737 MAX jets were introduced into service in 2017.