Police Chief James Craig says a private security guard had briefly left a checkpoint when a bag holding a gun moved through an X-ray machine.
According to police, the man passed through security before guards discovered a grainy X-Ray image of a possible gun in his bag.
Jeff Kulman, 38, of Taylor, said he was on the 15th floor when people were told to evacuate.
"All we know now is (the suspect is) described as male, white, dark suit, and we'll get to the bottom of it", Craig said.
Police and several other law enforcement agencies, including the Federal Bureau of Investigation, spent all of Monday searching for a man who was believed to have a gun in the building after it was photographed during a security scan.
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Thomas Jeffs told FOX 13 he helped his father stay hidden from federal authorities when Warren Jeffs was a fugitive from the FBI. However, Jeffs' attorney Kathryn Nester insisted it was against her client's constitutional rights to be jailed without trial.
"A crime was still committed", Woody said. Woody says police still were trying to identify him.
The AP was unable to reach Securitas officials in Detroit late Monday afternoon.
"Officers made contact with this individual who met detectives at his attorney's office and he then provided a statement and was interviewed", Woody said.
Craig stressed that the man is a person of interest, someone they want to investigate further, not a suspect.
The incident began after someone passed through security Monday morning and went into the building with what appeared to be a gun in some type of carry bag.
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He said he was at the Millennium Stadium that night with his father and friends and still remembers the occasion vividly. Gareth Bale plans to have an even greater influence at Euro 2016 and fire Wales into the knock-out stages.
Andrew Sokoly, a member of Councilman Scott Benson's staff, told the Detroit Free Press he was in his office with a few other staffers a little before 9:30 a.m. when an announcement over the public address system said to stay inside and lock the door.
There are numerous police cars and officers around the Coleman A. Young Municipal Building and the SWAT team has been called in.
"I felt pretty secure", Sokoly said.
The mayor's office says Detroit's City Hall will be closed for the rest of the day after a man entered an adjacent county court building with what might have been a handgun in a small briefcase.
Melissa Thomas, 41, of Westland was in the building with her grandson when they were evacuated.
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The stepdad of an 11-year-old girl New Hampshire who went missing five years ago has been charged in her 2011 murder. And I can search you right in the eyes and tell you the truth. "I still think there is more to it", he said.