The crew of an Indonesian passenger jet that crashed off Jakarta at the weekend with 62 people aboard did not declare an emergency or report technical problems before it suddenly plunged into the sea, an investigator said Monday.

Authorities have so far been unable to explain why the 26-year-old plane crashed just four minutes after takeoff, but say they've pinpointed the location of the black boxes.

A recording of conversations with air traffic control pointed to routine exchanges, and there was no communication as the Sriwijaya Air Boeing 737-500 plunged about 10,000 feet (3,000 metres) in less than a minute before slamming into the Java Sea, said National Transportation Safety Committee investigator Nurcahyo Utomo.

-It's like a normal conversation and nothing suspicious- he told AFP.

-There's no talk of an emergency or something like that.-

The preliminary data suggested it was "most likely" the plane was intact when it hit the water Saturday, he added.

-But we don't know at this stage- what caused the crash, Utomo said.

Switched flight

There were 62 Indonesian passengers and crew aboard the half-full flight, including 10 children.

The jet's captain, Afwan -- a 54-year-old father of three who like many Indonesians goes by one name -- was a former air force pilot with decades of flying under his belt, according to local media.

Some of the 2,600 personnel working in the recovery effort involving dozens of boats and helicopters are hauling body parts, twisted piece of wreckage and passengers' clothing from shallow waters about 23 metres (75 feet) deep.

Body bags filled with human remains were being taken to a police morgue where investigators hope to identify victims by matching fingerprints or DNA from their remains to living relatives.

Rapin Akbar, who gave a blood sample to the hospital, had five relatives on board including an older sister, a nephew and his wife and their seven-month-old baby.

They were flying back to Pontianak, the city on Indonesia's section of Borneo island, about 90 minutes away.

-(My nephew) had planned to go back to Pontianak on Sunday but changed his mind and decided to fly on Saturday instead- Akbar told AFP.

-He called me to say the flight was delayed and sent me a picture of their baby. It was (their) first.-

Late Monday, authorities identified flight attendant Okky Bisma, 29, as the first confirmed victim after matching fingerprints from a retrieved hand to those in a government identity database.

Black box data

Divers searched waters off Jakarta for black boxes -- cockpit voice and flight data recorders -- that could be crucial to help explain why the plane went down.

Despite the name, black boxes are usually bright orange with reflective stripes, and all commercial planes are obliged to have them on board.

Built to survive at vast depths and in extreme heat, they are fitted with a beacon which can emit a signal for a month.

The devices record information about the speed, altitude and direction of the plane as well as flight crew conversations.

Black box data help explain nearly 90 percent of all crashes, according to aviation experts.

The probe into Saturday's crash is likely to take months.

Aviation analysts said flight-tracking data showed the plane sharply deviated from its intended course before it went into a steep dive, with bad weather, pilot error and mechanical malfunction among the potential factors.

-Something quite dramatic has happened after takeoff- said Stephen Wright, professor of aircraft systems at Finland's Tampere University.

-The airspeed is far too low. The aircraft didn't accelerate up to the correct speeds for continuous flight.-

Spotty safety record

Sriwijaya Air, which flies to destinations in Indonesia and Southeast Asia, has said little about the plane, which was previously flown by US-based Continental Airlines and United Airlines.

The Indonesian carrier has not recorded a fatal crash since it started operations in 2003.

But the nation's fast-growing aviation sector has long been plagued by safety concerns, and its airlines were once banned from entering US and European airspace.

In October 2018, 189 people were killed when a Lion Air Boeing 737 MAX jet crashed near Jakarta.

That accident -- and another in Ethiopia -- saw Boeing hit with $2.5 billion in fines over claims it defrauded regulators overseeing the 737 MAX model, which was grounded worldwide following the accidents.

The 737 model that went down Saturday was first produced decades ago and was not a MAX variant.

In 2014, an AirAsia plane headed from Surabaya to Singapore crashed with the loss of 162 lives.

A year later more than 140 people, including scores on the ground, were killed when a military plane crashed shortly after take-off in Medan on Sumatra island.