One of the pilots of a jet that crashed in the Alps killing all 150 people on board was locked out of the cockpit before it started its descent, according to US media.

Investigators have not revealed details of conversations on the cockpit voice recorder black box recovered from the crash site.

But the New York Times quoted a senior military source involved in the probe as saying one of the two Germanwings pilots appeared to have left the cockpit and then been unable to get back in.

The source said: “The guy outside is knocking lightly on the door and there is no answer. And then he hits the door stronger and no answer.

“There is never an answer. You can hear he is trying to smash the door down.”

The official added: “We don’t know yet the reason why one of the guys went out.

“But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door.”

The source said conversation between the pilots during the early part of the flight from Barcelona to Dusseldorf had been “very smooth, very cool.”

In January a Delta Air Lines flight was forced to make an emergency landing after a pilot was accidentally locked out of the cockpit.

The door malfunctioned leaving the first officer to land the plane at McCarran International Airport in Las Vegas.

Cockpit doors have been reinforced since the September 11 attacks in 2001, to stop intruders from reaching the crew and controls.

The New York Times report emerged after a bus carrying the first relatives to the area where their loved ones lost their lives left Barcelona on Wednesday evening.

The group of 14 will meet with other family members who chose to fly from Barcelona to Marseille this morning.

The crash investigation is likely to focus on why there was no distress call from the plane, which went into a slow descent shortly after reaching its 38,000ft cruising altitude and continued along its flight path for some eight minutes before smashing into mountains with such force that it was “pulverised”, according to one official.

Theories include the complete incapacity of the cockpit crew, possibly after a windscreen blow-out.

Asked about reports that the second black box – the flight data recorder which indicates how an aircraft’s systems were working – had been found but was too badly damaged to be of use, a French air investigation bureau spokesman said this was not the case.

“We have not located the second black box,” he said.

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