The Boeing 737 MAX 8 literally fell out of the sky near where the two men were fishing about 15 km (9 miles) off the coast, silently at first and then with a deafening crash as it smacked into the sea.
The plane dropped more than 500 feet (152 metres), veered to the left and then started climbing again to 5,000 feet (1,524 metres). It gained speed in the final moments before data was lost when it was at an altitude of 3,650 feet (1,113 metres).
The Boeing 737 MAX 8 is the most recent model of Boeing's famous 737, the US company's best-selling plane, and is a popular choice among budget airlines around the world.
Lion Air Chief Executive Edward Sirait said on Monday that the plane had encountered an unspecified "technical issue" on its previous flight, which was from the resort island of Bali to Jakarta, but this had been "resolved according to procedure".
"We don't dare to say what the facts are, or are not, yet," he told reporters. "We are also confused about the why, since it was a new plane."
The captain of Monday's flight JT610 from Jakarta to Pangkal Pinang, the main town on Bangka, a beach-fringed island off Sumatra, was Bhavye Suneja, a 31-year-old Indian citizen originally from New Delhi. He and an Italian passenger were the only known foreigners on board.
According to his Linkedin account, Suneja had worked for Lion Air since 2011, clocking up some 6,000 flight hours. On Facebook there are photos of him in his Lion Air uniform, smiling.
Minutes after take-off at 6:20 am, Suneja reported technical difficulties and obtained permission from ground officials to turn back.
Data from FlightRadar24 showed the first sign of something amiss was around two minutes into the flight, when the plane had reached 2,000 feet (610 metres).