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Yep, it's true.

In an instantly viral Details interview, Shia LaBeouf spoke at length about a supposed onset fling with Megan Fox, his former costar in the first two Transformers flicks.

"Yes, Megan and Shia hooked up," a set source confirms to Us Weekly. In his Details chat, LaBeouf, 25, sputtered when asked if Fox was then involved with her now-husband Brian Austin Green during their tryst (he repeated "I don't know" 12 times).

But the kissing costars are in the clear, the source says. "It was when Megan and Brian broke up," the source explains. "They had a 10-month break before getting back together." (Indeed, Fox and Green, 36, did have a break of nearly a year before reuniting and eventually tying the knot in June 2010.)

"[Shia and Megan] bonded while filming," the source explains of the epic shoot, which was worsened by director Michael Bay's rumored temper. "Believe me, that was an unhappy set. I think they were drawn together because it was so ugly working on that film."

Although Bay has said that Fox was fired (at Steven Spielberg's demand) from the flicks for public comments she made about him, "she was never fired," the source insists. "She said enough was enough and quit."