PARIS — The co-pilot of Germanwings flight 9525 deliberately crashed the aircraft, French officials said Thursday, pointing to voice recorder evidence that he had locked the captain out of the cockpit, ignored his pleas for re-entry and steered down into the French Alps as passengers were heard screaming.

The assertions instantly changed the nature of the Tuesday crash, which obliterated the Airbus A320 and killed all 150 aboard, into a wide-ranging criminal investigation that focused on the co-pilot, a 27-year-old German with no obvious reason to commit mass murder, who had been hired less than two years ago. The top executive of Lufthansa, the parent company of Germanwings, said he was speechless at the news from France.

In Marseille, the chief prosecutor handling the criminal investigation, Brice Robin, said, “At this moment, in light of investigation, the interpretation we can give at this time is that the co-pilot through voluntary abstention refused to open the door of the cockpit to the commander, and activated the button that commands the loss of altitude.”

Mr. Robin said it appeared that the intention of the co-pilot, identified as Andreas Lubitz, had been “to destroy the aircraft.” He said the voice recorder showed that the co-pilot had been breathing until before the moment of impact, suggesting that he was conscious and deliberate in bringing the plane down and killing 144 passengers and five other crew members on Tuesday.

The German Foreign Ministry said half the dead were Germans, including four dual citizens. Most of the others were from Spain, with a smattering from other countries. Three were from the United States.

The revelation that one of the pilots of the jetliner was locked out of the cockpit before it crashed, which was first reported in The New York Times, raised troubling questions. Search teams continued to scour the rugged terrain of the French Alps for clues, and several other issues remained unclear on Thursday, including the identity of the captain and why he had left the cockpit.

The inquiry shows that the crash was intentional, Mr. Robin said, and said he was considering changing his investigation from involuntary manslaughter to voluntary manslaughter.

He said there was no indication that it was a terrorist attack, and added that Mr. Lubitz was not known to law enforcement officials. After the news conference, the German interior minister, Thomas de Maizière, told reporters in Berlin that security officials had checked their records after Tuesday’s crash and found no indication that anyone aboard had links to terrorism.

An investigation into the background of Mr. Lubitz, who was from the German town of Montabaur, is underway. German news media initially said he was 28, but the local authorities said he was born on Dec. 18, 1987, which would make him 27.

Asked if Mr. Lubitz had tried to commit suicide, the French prosecutor said, “I haven’t used the word suicide,” adding that it was “a legitimate question to ask.”

The flight was heading to Düsseldorf, Germany, from Barcelona, Spain, on Tuesday morning when it descended and slammed into the French Alps.

The French prosecutor said that the authorities had a full transcript of the final 30 minutes of the voice recorder.

“During the first 20 minutes, the pilots talk normally,” Mr. Robin said, saying they spoke in a “cheerful” and “courteous” way. “There is nothing abnormal happening,” he said.

The prosecutor said the transcript showed that the captain was preparing a briefing for landing in Düsseldorf. The co-pilot’s answer, the prosecutor said, was “laconic.”

The commanding pilot then asks the co-pilot to take over, and the noise of a seat backing up and a door closing can be heard.

“At this stage, the co-pilot is in control, alone,” the prosecutor said. “It is when he is alone that the co-pilot manipulates the flight monitoring system to activate the descent of the plane.” The prosecutor said that this action could only have been “voluntary.”

The control the co-pilot activated requires several turns, Mr. Robin said, and it could not have been turned to such a low altitude accidentally.