PARIS — French accident investigators said Monday that they had succeeded in downloading all of the flight data and cockpit conversations from the so-called black boxes of an Air France jet that crashed two years ago in the Atlantic Ocean — a critical breakthrough that could finally resolve the mystery behind why the plane went down.

Investigators at the French Bureau of Investigations and Analyses spent a full day painstakingly removing, drying and testing the circuits of the flash memory chips inside the flight recorders, which arrived Thursday at the agency’s headquarters in Le Bourget, near Paris. The data and the voice recordings were then successfully downloaded over the weekend and transferred onto a secure computer server. Copies were made and provided to the French judicial police, who are conducting a separate criminal inquiry into the crash.

The plane’s flight data recorder tracks roughly 1,300 different statistics, including the plane’s position, speed, altitude and direction when it began to experience difficulties. Investigators plan to synchronize the data with the voice recorder, which includes the final two hours of the pilots’ conversations and other cockpit sounds, including any alarms that would have sounded as its flight systems failed.

In a statement, investigators said they would spend the next several weeks conducting a detailed analysis of the black box recordings in order to assemble a fuller narrative of what happened in the flight’s final moments. An interim report on their findings was expected to be published during the summer, the agency said.