FBI Screening Mail Sent to 'Empire' Cast After Jussie Smollett Attack

The FBI is working with folks on the "Empire" set to track down the person who sent Jussie Smollett a hateful, threatening letter just days before the attack, and they're also on high alert to prevent dangerous mail and other items from making their way to the cast.

Sources on set tell us a private security company was hired by the show and met with FBI agents to establish a protocol for inspecting all incoming mail. We're told the team wears gloves so the evidence in question is not contaminated.

The security team has 2 functions ... to flag threatening, incoming mail and alert the FBI, and to possibly link new items to the letter Jussie already received, containing homophobic and racists epithets along with an image of a noose.

Our sources say the security team has already flagged "hate mail," but we're told that it's nothing new. The show has been targeted by racists and homophobes in the past, but now everyone is on high alert and the FBI is screening all of it.

We're told there are also daily security sweeps on the "Empire" set. Our sources say guards are looking for anything or anyone who might be out of place.

Production hired two additional makeup artists over the weekend to assist Jussie and make sure his cuts and bruises from the Chicago attack are well-hidden.

As we reported -- Jussie returned to Chicago Sunday after his show in L.A. -- and cops want to talk with him again as a part of their ongoing investigation.