Police have not identified nor charged the suspect. Officials did not release any details about the alleged threat.
Authorities seized guns and gunpowder from the suspect's apartment in the 4200 block of North Sheridan Road Monday afternoon. Police evacuated two floors of the apartment building while they removed evidence.
As the tallest building in North America, the Willis Tower has been the focus of increased security concerns, particularly since the death of Osama bin Laden. Some have predicted a terrorist response. But neighbors of the suspect say he fails to fit the profile of a terrorist.
"He's been a really great neighbor for me. He's dug me out of many, many, many a horrible snow catastrophe. He's been great," said Jaela Neal, neighbor.
The suspect is being held at the Chicago Lakeshore Hospital where he is undergoing a psychological evaluation.
The Chicago police and the Joint Terrorism Task Force are investigating.