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(CNN) A letter containing a white powder was sent to the Chicago set of "Empire" days before one of the show's stars, Jussie Smollett, was allegedly attacked by two men on January 29.

Chicago police spokesman Anthony Guglielmi told CNN that the letter, received on January 22 at Cinespace Studios, where the show is filmed, prompted a response from the HAZMAT unit.

Guglielmi said authorities determined the powder to be aspirin, but declined to give details on the content of the letter.

The FBI is leading the investigation into the letter, he added.

A spokesperson for 20th Century Fox Entertainment, which produces the series, declined to comment when reached by CNN.

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