Taylor Swift's song "Bad Blood" is, as many fans speculated, about her rivalry with Katy Perry, not Miley Cyrus, sources tell Us Weekly Credit: Jim Spellman/WireImage; Jun Sato/WireImage; Jason Merritt/Getty Images

You're gonna hear them roar. Ever since Taylor Swift told Rolling Stone that she'd written a song, "Bad Blood," about her rivalry with a fellow female artist, speculation has been rampant about who the artist in question is. Some have mentioned Miley Cyrus as a possibility, but an overwhelming number of fans think the track is a dig at Katy Perry — and sources tell Us Weekly those fans are right.

Perry herself fueled the speculation when she tweeted about an unnamed mean girl on Tuesday, Sept. 9, the day after Rolling Stone's story went live. "Watch out for the Regina George in sheep's clothing..." the "Roar" songstress, 29, wrote, referring to Rachel McAdams' character in Mean Girls.

"Katy knows Taylor's song is about her, which is why she tweeted that," one insider tells Us, noting that both stars can be "very competitive." The source adds that the fellow chart-toppers used to be at least amicable, but that a real friendship between them never flourished.

Indeed, Swift, 24, told Rolling Stone that she wasn't really sure where she stood with the unnamed girl in "Bad Blood" until pretty recently.

"For years, I was never sure if we were friends or not," the "Shake It Off" singer explained. "She would come up to me at awards shows and say something and walk away, and I would think, 'Are we friends, or did she just give me the harshest insult of my life?' [Then last year] she did something so horrible. I was like, 'Oh, we're just straight-up enemies.' And it wasn't even about a guy!"

That may not be entirely true. Both stars dated John Mayer, and Swift admitted later in her Rolling Stone interview that there may have been a "personal" element to the conflict. "But I don't think there would be any personal problem if she weren't competitive," she told the mag.

The main source of tension, she said, was business. "She basically tried to sabotage an entire arena tour," Swift alleged. "She tried to hire a bunch of people out from under me."

Those people, the first source tells Us, were her dancers — who worked for Perry first. "They were loyal to Katy when it came time to launch the Prism tour," the insider says. "She didn't steal them. Taylor just employed them when they weren't working for Katy."

Fans who speculated about Swift's relationship with Cyrus aren't necessarily wrong about them not getting along, but the "Wrecking Ball" singer "didn't steal [Taylor's] tour people," a second source tells Us. "She's not the focus of the song." That said, the source adds, "Miley is competitive with Taylor." (As some fans have noted, Cyrus looked unimpressed during Swift's performance at the VMAs this year.) See pics of the celebs on Taylor’s good side.