Audio from the cockpit voice recorder seems only to deepen the mystery surrounding the crash and provides no indication of the condition or activity of the pilot who remained in the cockpit.

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PARIS — As officials struggled Wednesday to explain why a jet with 150 people on board crashed in relatively clear skies, an investigator said evidence from a cockpit voice recorder indicated one pilot left the cockpit before the plane’s descent and was unable to get back in.

A senior military official involved in the investigation described “very smooth, very cool” conversation between the pilots during the early part of the flight from Barcelona, Spain, to Düsseldorf, Germany. Then the audio indicated one of the pilots left the cockpit and could not re-enter.

“The guy outside is knocking lightly on the door and there is no answer,” the investigator said. “And then he hits the door stronger and no answer. There is never an answer.”

Eventually, he said: “You can hear he is trying to smash the door down.”

While the audio seemed to give some insight into the circumstances leading up to the Germanwings Flight 9525 crash, it left many questions unanswered.

“We don’t know yet the reason why one of the guys went out,” said the official, who requested anonymity because the investigation is continuing. “But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door.”

Since the Sept. 11 terrorist attacks, airlines in the U.S. don’t leave one pilot alone in the cockpit. The standard operating procedure is that if one of the pilots leaves — for example to use the bathroom — a flight attendant takes their spot in the cockpit. It was not immediately clear if European airlines have adopted the same practice.

The names of the pilots have not been released.

The data from the voice recorder seems only to deepen the mystery surrounding the crash and provides no indication of the condition or activity of the pilot who remained in the cockpit. The descent from 38,000 feet over about 10 minutes was alarming but gradual enough to indicate that the twin-engine Airbus A320 had not been damaged catastrophically. At no point during the descent was there any communication from the cockpit to air traffic controllers or any other signal of an emergency.

When the plane plowed into craggy mountains, it was traveling with enough speed that it was all but pulverized, killing the 144 passengers and crew of six and leaving behind almost no apparent clues about what caused the crash.

The French aviation authorities have made public very little, officially, about the nature of the information that has been recovered from the audio recording, and it was not clear whether the recording was partial or complete. France’s Bureau of Investigations and Analyses confirmed only that human voices and other cockpit sounds had been detected and would be subjected to detailed analysis.

Asked about the new evidence revealed in the cockpit recordings, Martine del Bono, a bureau spokeswoman, declined to comment.

“Our teams continue to work on analyzing the CVR,” she said, referring to the cockpit voice recorder. “As soon as we have accurate information, we intend to hold a press conference.”

At the crash site, a senior official working on the investigation said workers found the casing of the plane’s other black box, the flight data recorder, but the memory card containing data on the plane’s altitude, speed, location and condition was not inside, apparently having been thrown loose or destroyed by the impact.

The flight’s trajectory before the crash also left many unanswered questions.

Rémi Jouty, director of the French Bureau of Investigations and Analyses, said at a news conference that the plane took off at 10 a.m. local time from Barcelona and that the last message sent from the pilot to air traffic controllers had been at 10:30 a.m., which indicated that the plane was proceeding on course.

But minutes later, the plane inexplicably began to descend, Jouty said. At 10:40 and 47 seconds, the plane reported its last radar position, at an altitude of 6,175 feet. “The radar could follow the plane until the point of impact,” he said.

Jouty said the plane slammed into a mountainside and disintegrated, scattering debris over a wide area and making it difficult to analyze what had happened.

One of the main questions outstanding is why the pilots did not communicate with air traffic controllers as the plane began its unusual descent, suggesting that either the pilots or the plane’s automated systems may have been trying to maintain control of the aircraft as it lost altitude.

Among the theories that have been put forward by air-safety analysts not involved in the investigation is the possibility that the pilots could have been incapacitated by a sudden event such as a fire or a drop in cabin pressure.

Jouty said it was too early in the investigation to speculate about possible causes.

Speaking on the French radio station RTL, Interior Minister Bernard Cazeneuve said Wednesday that terrorism was not a likely “hypothesis at the moment,” but that no theories had been excluded.

Lufthansa, the parent company of Germanwings, has characterized the crash as an accident. The airline has not disclosed the identities of the pilots, except to say that the captain was a 10-year veteran with more than 6,000 hours of flying time in A320s.

Germanwings Chief Executive Thomas Winkelmann said the dead included 72 Germans, 35 Spaniards, three Americans and two people each from Australia, Argentina, Iran, Venezuela, and one person each from Britain, the Netherlands, Colombia, Mexico, Japan, Denmark, Belgium and Israel.

The three Americans included a mother and daughter, Yvonne Selke, 58, of Nokesville, Va., and her daughter, Emily Selke, 22, the U.S. State Department said. It declined to identify the third American. Some of the victims may have had dual nationalities; Spain’s government said 51 citizens had died in the crash.