TOULOUSE, France – Investigators probing the crash of a Germanwings jetliner in the foothills of the French Alps will seek to determine why the aircraft made a rapid descent spanning eight minutes without a single mayday call.

It was French air traffic controllers, not the A320’s two pilots, who declared an emergency after the plane spent just 60 seconds cruising at 38,000 feet before plunging, seemingly still under control, to 5,000 feet. The Airbus Group model, bound for Dusseldorf from Barcelona, was carrying 150 people.

Why neither pilot issued a distress call and why the plane lost altitude without seeking permission will be a key focus for France’s BEA air-accident investigation bureau as it probes the crash and the plane’s black boxes, once of which has been found. Declaring an emergency even in difficult circumstances is a bedrock principle of pilot training, compounding the mystery over the crew’s silence as the tragedy unfolded in broad daylight and benign weather conditions.

“Something triggered a descent,” said Paul Hayes, director of safety at London-based Ascend, an aviation data provider. “They’ll want to know whether that’s to do with the aircraft, with some external event, with the crew, or with a combination of all of those. And they’ll want to know why the descent wasn’t stopped and the aircraft recovered.”

Germanwings said that the A320 was delivered to its parent, Deutsche Lufthansa, in 1991 and was flown by a pilot with 6,000 hours of experience on Airbus jets. The plane was serviced last summer, had logged 46,700 flights, and flew to Barcelona Tuesday morning without incident, the airline said.

French controllers declared an emergency at 10:47 a.m. as they saw the narrow-body rapidly losing height over the Alpes de Haute-Provence region, said Eric Heraud, a spokesman for French civil aviation authority DGAC. Attempts were made to contact the crew during the descent, without success, according to the SNCTA union, which represents air traffic management workers.

“There were a lot of consecutive calls,” SNCTA national secretary Roger Rousseau said. “The controller sees the altitude of the aircraft on his radar, and if he sees the altitude dropping he immediately calls the pilot.”

The majority of air crashes occur upon takeoff or landing, with airlines and aviation regulators having steadily eliminated causes such as mid-air collisions.

While a number of developments could cause an aircraft to make a rapid descent from cruising altitude, such as a double engine failure tied to contaminated – or insufficient – fuel, under such circumstance the pilots would ordinarily still be able to communicate their predicament.

More-catastrophic events might include a cracked cockpit windscreen, a sudden loss of the aircraft’s integrity from faults with its fuselage or wings, or a malfunction of flight systems including those governing external communications.

Bill Waldock at the Embry Riddle Aeronautical University in Prescott, Arizona, said that the Germanwings A320’s eight-minute descent was “fast, but not outside the limits,” and that the silence of its pilots may suggest they were too preoccupied with getting the plane to a lower altitude to talk.

“No communication would argue something was going on which caused the pilots to not really try to communicate,” he said. “If they had an emergency of some sort and were trying to control the airplane, that’s the number one priority.”

Waldock, who has taught accident investigation for 25 years, characterized the loss of altitude as consistent with “some sort of controlled descent” that may point to a cabin- pressurization failure rather than a stall event, in which an aircraft plummets after losing lift.

With the Germanwings A320 appearing to have hit the ground largely intact and with some forward momentum, the plane seems unlikely to have been blown apart by a bomb, and there’s no evidence of a hijacking.

It’s still possible that the jet was brought down by an intentional act, said Steve Wallace, former head of the Federal Aviation Administration’s accident-investigation arm. The most credible explanation of the disappearance of a Malaysian Air jet last year is that someone in the cockpit sent it off course, he said, adding that investigators will want to study similar theories in probing Tuesday’s tragedy.

For now, the BEA and Airbus have sent teams to the scene of the crash to begin sifting through evidence. France’s minister for the interior, Bernard Cazeneuve, has also gone to the site, French President Francois Hollande said on television.

John Cox, the head of aviation safety consulting firm Safety Operating Systems in Washington, said the debris field is likely to provide clues as to how the A320 met its end, with the survival only of smaller pieces of wreckage indicating a higher-speed impact.

The sequence of events should be made clearer by analysis of the plane’s voice and data recorders, which in addition to providing a wealth of readings on the flight itself should reveal what exchanges took place between the pilots – or with air traffic control – in the critical minutes before the crash.

“They’re going to be looking for the black boxes very strongly,” Cox said. “You look at everything. The weather, the crew, the maintenance history, the airplane history.”

One of the flight recorders has been recovered, the French government said, without specifying which one.