Brazilian authorities have confiscated the passports of the two US pilots at the helm of a private jet that collided in mid-air with a commercial airliner.
Investigators hope the black box recorders will reveal what happened
The pilots were unhurt after making an emergency landing, but were ordered to stay in Brazil during investigations.
Reports suggest the small plane was flying at 37,000ft (11,300m), the same height as the passenger jet.
The airliner, a Boeing 737 carrying 154 passengers and crew, crashed into the Amazon rainforest killing all on board.
An initial death toll of 155 was revised downwards by the airline, Gol, after it was found that one passenger had been listed twice.
A judge in the state of Mato Grosso, where both planes came down, said both pilots had been asked to stay in Brazil during preliminary investigations.
"If they returned to the United States it would require a lot of time and effort for us to collect their testimony," he told the Reuters news agency.
General Luis Carlos Bueno, commander of Brazil's air force, said that one of the planes must have deviated from its designated flight plan.
"The Boeing had to stay at an altitude of 37,000ft and the executive plane at 36,000ft," he said.
"What we are unable to confirm is which one left its route."
An American journalist travelling on the private plane, Joe Sharkey, wrote in the New York Times on Tuesday that he had noted down the altitude reading - 37,000ft - while in the cockpit moments before impact.
The collision was the worst air disaster in Brazilian history.
Flight recorders from both planes have been recovered and are being examined by accident investigators.