Federal prosecutors in Manhattan investigating the disgraced movie mogul Harvey Weinstein have for months been looking into his dealings with Black Cube, a private Israeli intelligence firm that was said to have been used to spy on some of the women who have accused him of sexual assault, a person familiar with the matter said on Thursday.

The inclusion of Mr. Weinstein’s ties to Black Cube added a previously unknown element to the inquiry and served to heighten the legal risks he was already facing. Black Cube’s involvement in the investigation was first reported by The Wall Street Journal.

After a decades-long reign as a hitmaker in Hollywood, Mr. Weinstein was charged in May by the Manhattan district attorney’s office with raping one woman and forcing another to perform oral sex. Two months later, the same prosecutors brought new charges against him, saying he had forced a third woman to have sex with him. Mr. Weinstein has also been accused in a series of civil lawsuits of sexual and physical assault — and of waging smear campaigns against some of his accusers.

The federal investigation started last year when prosecutors from the United States attorney’s office in Manhattan began to examine whether Mr. Weinstein had committed fraud when he arranged for two auction items — a sitting with a fashion photographer and a package of tickets to a Hollywood awards event — to be offered together at an AIDS charity fund-raiser in France in May 2015. There was one condition to the deal, said people with knowledge of the matter: $600,000 of the proceeds had to go to a theater that had staged a trial run of a Broadway musical produced by Mr. Weinstein.