Chicago Police Spokesman Anthony Guglielmi told CNN that the letter, received on January 22nd at Cinespace Studios, where the footage was shot, prompted a response from the HAZMAT unit.
Guglielmi said authorities have determined that dust is aspirin, but refused to give details about the content of the letter.
The FBI is investigating the letter, he added.
A spokeswoman for 20th Century Fox Entertainment, who produces the series, declined to comment when he arrived at CNN.
In Chicago, the police are continuing an investigation into the alleged assault, which took place near the lower entrance to the Loews Chicago Hotel.
Smollett said that attackers physically attacked him and called him homophobic and racist insults, the police said.
Smollett, who was returning from an early morning excursion to a sandwich shop in the subway, also reported that one of his attackers had put a rope around his neck and poured out an unknown substance to him, the police said.
Investigators have a video of Smollett entering the hotel after allegedly attacking what appears to be a hinge around the neck, Guglielmi said.
The police have a rope and a Smollett sweater was worn on the night of the attack and testing the substance, Guglielmi says.
Smollett and his manager, Brandon Moore, told the authorities to talk on the phone at the time of the attack, the police said.
Guglielmi said that because of the financial relationships between these two men, they are trying to confirm that the call was executed and that they asked for the phone or physical phones used for the call.
The police were called 30-40 minutes after the attack, said Guglielmi.
Detectives have collected and watched more video surveillance from neighboring hotels and apartments, but at this moment they did not find a video of the incident, said Guglielmi.
On Wednesday, the police released photographs of the two people now requested for a case-related inquiry.
Smollett acted on Fok's "Empire" from 2015. He plays Jamal, a successful singer in the Lyon music family. Like his character, Smollett is identified as gay.
On Thursday, the Smollett family condemned the "violent and unprovoked attack," calling it "racial and homophobic hate crime" in its first official statement of the incident.
"Jussie told the police everything from the start," the statement said. "His story has never changed, and we hope to find these people and bring them to justice."
After Smollett's attack, his friends, colleagues and colleagues showed the support of an actor who is an active advocate of the LGBTK community.
"Our family thanks everyone for their prayers and for the enormous amount of love he received. We are grateful to our village for your tremendous support during this difficult time," the statement added. "We are so grateful that God saw him through this cowardly attack alive. Jussie is a warrior whose light can not be dimmed."