NEW YORK/LOS ANGELES (Reuters) - Britney Spears and former Disney Channel star Demi Lovato officially joined the U.S. version of "The X Factor" on Monday, leading creator Simon Cowell to pledge the second season would "seriously kick butt" when it returns in September. Spears, 30, the world's biggest pop phenomenon of the 2000s, and Lovato, 19, ...

Mel B, Anastasia, Nicole Scherzinger and Sharon Osbourne also in the running to replace Kelly Rowland

The team at the US version of The X Factor cleaned house on Monday

Nicole Scherzinger ran off the set of 'X Factor' in tears Thursday night

"X-Factor" cast-off Drew Ryniewicz is lashing back after some deranged fans threatened to kill Paula Abdul and Nicole Scherzinger last week