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Selena Gomez, Nat Wolff

Sam Sharma/ Miguel Aguilar, PacificCoastNews.com

Justin Bieber and Selena Gomez's breakup wasn't exactly a shock to those in their respective inner circles. In fact, multiple sources tell E! News the split was "a long time coming" and jealousy had long been building up between the duo.

Last week, Jelena fans pointed the finger at Victoria's Secret model Barbara Palvin, after she was snapped with the Biebs in New York. But some of those close to the ex-couple tell E! News that it was another photo taken months earlier that had a greater impact and helped turned their romance rocky.

It was an on-set picture of Selena smooching her Parental Guidance costar Nat Wolff taken in August.

The particularly passionate kiss between Gomez and the Naked Brothers star was purely for show—they were in character for the film.

Still...

Barbara Palvin, Justin Bieber Twitter

Twitter

"[Justin] freaked out on that," says one insider close to the former couple. "He would get jealous and she wouldn't tolerate it."

A second intimate source couldn't specifically say whether the photograph was the start of their relationship falling apart but did say that "Justin was very jealous of it. He knows it was a photo of them filming, but it still made him jealous."

There was no comment from reps for either star. But our sources say the jealousy worked both ways.

"They're young. Of course they have their jealous streaks," the second source continues. "Justin has gotten jealous, and so has Selena."

Further complicating things: their hectic professional lives. "He is always working," says the source. "And she is always working."

Even now, in the wake of their split, both Justin and Selena moving forward as usual with a plethora of public appearances.  

On Monday night, Selena told E! News she is "doing well" before being honored at the Glamour Awards in New York, while the Biebs put on a brave face performing in Brooklyn. Meanwhile, Bieber was back in Los Angeles on Tuesday, getting ticketed by police, while Gomez traveled to Texas to appear with Ryan Seacrest at an event for the latter's foundation.

Busy, indeed.

(Originally published Nov. 15, 2012, at 5:30 a.m. PT)