EDIT - Since this story was first published, Singer has deleted his original message and re-tweeted the following:
Evan Peters, the breakout star of FX's "American Horror Story," has been cast as Quicksilver in "X-Men: Days of Future Past." The announcement came this afternoon from director Bryan Singer, who wrote on Twitter:
Before he was an #Avenger, he was just a REALLY fast kid. Thrilled to say #EvanPeters is joining #XMen #DaysOfFuturePast as #Quicksilver.
— Bryan Singer (@BryanSinger) May 23, 2013
Interestingly, director Joss Whedon confirmed just last week that Quicksilver and his sister the Scarlet Witch are set to appear in his sequel to "The Avengers." While Fox holds the film rights to the X-Men characters in perpetuity, Marvel Studios President Kevin Feige revealed in April 2012 that the mutant children of Magneto would be available to both studios, saying, "It's a little complicated, but if they [Fox] want to use them in the X-Men movie they could, [but] if we want to use them in 'The Avengers' movie, we could."
Whether Singer's inclusion of Quicksilver in "Days of Future Past" will move Whedon to change his plans remains to be seen. However, he's made it clear the "brother-sister act" are his favorite characters from the Avengers comics.
Peters, who appeared in 2010's "Kick-Ass," joins a sprawling cast that includes James McAvoy, Michael Fassbender, Nicholas Hoult, Jennifer Lawrence, Hugh Jackman, Ian McKellen, Patrick Stewart, Halle Berry, Anna Paquin, Ellen Page, Shawn Ashmore, Peter Dinklage, Daniel Cudmore, Omar Sy, Booboo Stewart, Fan Bingbing and Adan Canto.
"X-Men: Days of Future Past" opens July 18, 2014.