PARIS — One of the ugliest unsolved crimes of France’s long-ago, quasi-colonial war in Algeria was finally laid to rest on Thursday, as President Emmanuel Macron recognized that the French Army had tortured and killed a youthful antiwar intellectual in 1957.

The death in custody of Maurice Audin, a 25-year-old mathematician, has for decades been a symbol of the French Army’s brutality during the Algerian War, much as the My Lai massacre became for the United States’ war in Vietnam. But unlike My Lai, which led to prosecutions, the Audin affair was never investigated.

There have been books, films and furtive late-life declarations by aging officers, but the mystery has never been solved. And until now, France had never admitted that it used torture in Algeria.

For 61 years, Mr. Audin’s widow, Josette, has battled the French state to have her husband’s killing recognized as the murderous work of military torturers during a critical phase of the Algerian War.