German investigators later revealed that Mr. Lubitz had a history of severe depression dating from at least 2009, and that he had scoured the Internet for methods of committing suicide in the days before his final flight.

The French report indicates that Mr. Lubitz’s maneuvers were so fleeting that they went undetected by air traffic controllers, who had already given instructions for a moderate descent to 35,000 feet from 37,000 feet. The flight’s captain, Patrick Sondenheimer, also apparently did not notice the maneuvers, which occurred about 20 minutes into the flight and while he was out of the cockpit for roughly five minutes.

The report says the co-pilot briefly selected a target altitude of 100 feet several times by turning a dial knob on the control panel while the plane was already in its gentle descent. To activate the selection and accelerate the decline, he would have had to pull on the knob. The maneuvers, which he quickly reversed, lasted from a few seconds to up to three minutes during the captain’s absence and would not have been noticed by anyone on board.

French investigators did not try to explain Mr. Lubitz’s actions, but they were unambiguous about his role in the crash.

“We cannot presume what was going on in his mind,” Mr. Jouty said. “But based on all the information that we have gathered so far, we can affirm categorically that this crash was the result of an intentional act, a series of steps that, taken together, all point in the same direction.”

The report notes similarities between Mr. Lubitz’s control inputs on the two flights and, in its analysis of the second, ill-fated flight, supports the prosecutor’s initial conclusions. The co-pilot “intentionally modified the autopilot instructions to order the airplane to descend until it collided with the terrain,” according to the report.

“He did not open the cockpit door during the descent, despite requests for access made via the keypad, the cabin interphone and knocks on the door,” it said.