Did Brad Cheat? Inside Pitt's Alleged Fling With Sexy Costar Marion Cotillard Before Divorce

It seems Brad Pitt's "insane chemistry" with sexy Allied costar Marion Cotillard may have led to the demise of his marriage to Angelina Jolie.

As Star exclusively reported in May, insiders suspected there was "something going on" between the 52-year-old father of six and the younger French actress, 40, on the London set of the romantic drama earlier this year.

"They had insane chemistry right from the start, and many crew members believe they've been acting on it in private," a source claimed, noting that they shared "secret inside jokes that no one else gets" and "affectionate nicknames" for each other.

And now, Page Six is reporting that Jolie, 41, discovered that the two were cheating, leading to her shocking decision to file for divorce.

"She hired a private eye because she felt that he was fooling around with her on the set, and it turns out, he was. And that was the final straw," a source told the paper.

Jolie's hired help also learned that the perceived family man had been partying like a wild bachelor during filming overseas, sources claimed.

"The atmosphere off-set was full of hard drugs and Russian hookers, and Angie was told Brad got caught up in it," the insider said. "He's in the throes of some insane midlife crisis, and Angie is fed up."

As RadarOnline.com reported, Jolie cited "irreconcilable differences" in her divorce filing from her husband of two years and partner for more than a decade.

Claiming she was focused on protecting her six children, she requested full physical custody of the entire brood.