Justin Bieber is rumored to have fired off a series of angry text messages to onetime pal Niall Horan demanding answers about him going behind the star's back to take up with Selena Gomez.

Hollywood Life reports Bieber is convinced the One Direction singer broke the bro-code by seeking out his ex, particularly after it had been widely reported Bieber and Gomez were still spending time together when Horan made his move.

"Justin is still furious with Niall for going behind his back and stealing Selena from him," a source claimed. "That's how he sees it. At first he didn't want to contact Niall in case it was just like a short-lived fling, which still is not cool with Justin. But now that Niall and Selena seem to have a serious relationship, Justin wants to see his so-called friend in person and confront him."

Horan and Gomez were first spied together at the Jingle Ball in Los Angeles on Dec. 4. About two weeks before that, Bieber and the "Come Get It" singer were spotted hanging out together at the Montage Hotel, where Bieber serenaded her with a rendition of "My Girl" in the hotel lobby.

Word is Horan has not responded to any of Bieber's text and has been advised by Gomez not to do so.

In the midst of it all, the 21-year-old Bieber is rumored to be hooking up with "Keeping Up With the Kardashians" reality TV star Kourtney Kardashian.

Scott Disick's 36-year-old ex and the mother of his three young children has recently been seen hanging out with Bieber and, in one instance, was spied leaving his residence in the early hours of the morning.

Gomez is reported to have been outraged by the development, and she is even rumored to have reached out to Bieber via text message to make her feelings known to him.