Police Superintendent Garry McCarthy said a worker at a technological company entered the 17th-floor office and met with his CEO before pulling a gun. There was a struggle for the gun, and the CEO was shot twice before the gunman fatally shot himself, McCarthy said.
The 54-year-old victim was taken to a hospital in critical condition. The other man, described as the 59-year-old offender was pronounced dead at the scene.
Officers were called to the scene around 9:50 am (1350 GMT) and found two men on the floor in an office, police spokesman Martin Maloney said. The office is in the Bank of America building, two blocks from the Willis Tower, the country's second tallest skyscraper, and a block from the Chicago Board of Trade and the Federal Reserve Bank of Chicago.
Workers said they received emails from the building security at about 10 in the morning telling them there was a security situation in the lobby and to stay at their desks. A few minutes later, someone came over the intercom to tell them everything was clear.