Police found guns and explosives inside of the man's North Side apartment.
Willis Tower -- formerly Sears Tower-- has been known as a genuine potential terrorist target. As the tallest building in North America, it has been the focus of security concerns since the terrorist attacks of September 11. And now that Osama bin Laden has been killed, the threat of retaliation against one of Chicago's most famous landmarks is real.
The suspect was in police custody Tuesday for allegedly making a threat against the 110-story skyscraper. After authorities raided his apartment in the 4200-block of Sheridan Road on Monday, they discovered approximately 20 guns, two grenades and pounds of gun powder.
"It's kind of scary to know someone like that has that much weaponry in your neighborhood because I actually live in Andersonville," said Willis Tower visitor Joe Hardin.
People who work at the globally iconic building say they are not surprised about the possible threat. Others at the tower Tuesday morning told ABC7 Chicago it would not affect the way they conduct their lives.
"The threats, I think a lot of them either aren't real, or it's just crazy people making threats that they never substantiate into actual attacks or anything like that," said visitor Soraya Freeman.
"Anything can happen anywhere, and at least they caught him. And if it did happen, I am grounded in who I am spiritually. So, I'm good for it," said Willis Tower employee Nicole Taylor.
"I can't live in fear," said Dan Ungerleider, also an employee at Willis Tower. "I have to go on and do my day-to-day thing to take care of myself and take care of my family."
The 60-year-old male suspect is being psychologically evaluated at Chicago Lakeshore Hospital. His identity is not being made public.
Police determined there was no overt threat to Willis Tower or to the general public after they took him into custody.
The man's neighbors say the man lives alone with his dog and seems like a nice guy.
"He's been a really great neighbor for me. He has dug me out of many, many, a horrible, you know, snow catastrophe. He's been great," said area resident Jaela Neal.
Sources who know the man tell ABC7 that he has a history of mental illness.
Police evacuated two floors of the suspect's apartment building as a precaution while they removed evidence Monday.
A joint terrorism task force is investigating the case. No charges had been filed late Tuesday morning.