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Shia LaBeouf, Isabel Lucas

AP Photo/Dan Steinberg, Jeffrey Mayer/WireImage.com

One of the lingering mysteries of Shia LaBeouf's accident has been why actress Isabel Lucas was along for the ride, even though she had been dating Entourage's Adrian Grenier.

E! News has learned the reason Lucas, 23, wasn't with Grenier that night: He had allegedly been spending time recently with model Melissa Keller.

"Adrian goes back and forth dating Melissa and Isabel," claims a source. But since the crash, Adrian has been by Isabel's side, and we're told they haven't been apart since.

Just two days before the accident, Grenier, 32, was Keller's date for the former Sports Illustrated model's 30th birthday party at a friend's house in Echo Park.

"It was a really small group, he was chatting with her parents, who love him," says a source. "They were dancing all night and were so cute."

After Isabel's accident, which she walked away from virtually unscathed, Grenier has put his romance with Keller on the back burner and gone back to Lucas. "Isabel was shaken up pretty badly," says a source. "He's been so protective of her."