One of the main questions is why the pilots did not communicate with air traffic controllers as the plane began its unusual descent, which suggested that 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 a pilot was incapacitated by a sudden event such as a fire or a drop in cabin pressure.

A senior French official involved in the investigation, who spoke on the condition of anonymity, said that the lack of communication from the pilots during the plane’s descent was disturbing, and that the possibility that their silence was deliberate could not be ruled out.

“I don’t like it,” said the French official, who cautioned that his initial analysis was based on the very limited information currently available. “To me, it seems very weird: this very long descent at normal speed without any communications, though the weather was absolutely clear.”

Image The French aviation authorities have recovered an audio file from the cockpit voice recorder, but did not say whether it was partial or complete. Credit... Bureau d'Enquetes et d'Analyses, via Associated Press

“So far, we don’t have any evidence that points clearly to a technical explanation,” the official said. “So we have to consider the possibility of deliberate human responsibility.”

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

“At this moment, I have no beginning of a scenario,” Mr. Jouty said. However, he said there was not yet any evidence available that would support a theory of a depressurization or of a midair explosion.

Speaking on the French radio station RTL, Interior Minister Bernard Cazeneuve said Wednesday morning that terrorism was not a likely “hypothesis at the moment,” but that no theories had been excluded. He said the size of the area over which debris was scattered suggested that the aircraft had not exploded in the air but rather had disintegrated on impact.

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.

Image French emergency services resumed work on Wednesday near the crash site of a Germanwings jet. Credit... Peter Kneffel/European Pressphoto Agency

The French Bureau of Investigations and Analyses, which is leading the technical inquiry into the crash, sent seven investigators to the crash site Tuesday. They have been joined by their counterparts from Germany, as well as by technical advisers from Airbus and CFM International, the manufacturer of the plane’s engines.

Speaking on Europe 1 radio, Jean-Paul Troadec, a former director of the French air accident investigation bureau, said one of the big challenges for investigators would be to protect the debris at the crash site from inadvertent damage.

“We need to ensure that all the evidence is well preserved,” Mr. Troadec said, referring to the pieces of the plane littered across the steep slopes as well as to the remains of the victims. The identification of the victims will probably require matching DNA from the remains with samples from relatives.

The recovery effort will be a laborious task, given the state of the wreckage, the difficult terrain and the fact that the crash site is so remote that it can be reached only by helicopter.

Cabin depressurization, one of the possibilities speculated about on Wednesday, has occurred before, perhaps most notably in the crash of a Cypriot passenger plane in 2005 that killed all 121 people on board as it approached Athens. In that case, Helios Airways Flight 522, a slow loss of pressure rendered both pilots and all the passengers on the Boeing 737 jet unconscious for more than three-quarters of an hour before the aircraft ran out of fuel and slammed into a wooded gorge near Athens, the Greek capital.

Investigators eventually determined that the primary cause of that crash was a series of human errors, including deficient maintenance checks on the ground and a failure by the pilots to heed emergency warning signals.