One page detailing a key scene revealing the mystery of a plotline from the show's previously-aired second season was taken from the set during a shoot in New Orleans, Louisiana earlier this month, reported TMZ online.
It contains the answer to a storyline that kept viewers hooked during the show's 'Asylum' season in 2012.
"The theft and unauthorised distribution of our scripts is a very serious matter and we will take all appropriate steps necessary to vigorously protect our intellectual property," a representative for 20th Century Fox TV said.