Kevin's letter to Britney was the final straw

Everyone suspected that Britney Spears would eventually drop Kevin Federline. But it was surprising that she chose to do so so shortly after giving birth to their second child. Something must have happened between the pair to cause her end the marriage at that time. Us Weekly claims to know what it was.

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The tabloid is reporting that Kevin wrote Britney a scathing letter the week before she announced their split. The letter, which was handwritten, arrived after the couple had been arguing via text messages for a several days. "It mostly blamed Britney for all their problems," says a source close to the family of the letter.

Shortly after receiving the letter, Spears called her lawyer to move forward with the divorce. She signed the divorce petition right before she appeared on "Late Show with David Letterman." Says the insider, "That's why she looked so happy [during the appearance]. A weight had been lifted."