"Kind of bizarre behavior," McCarthy said. "I don't know what his motivation was."
Even though the man had a valid firearms owner identification card, the weapon he used was stolen from Indiana, McCarthy said. The suspect was taken into custody a short time later, just a few blocks away. No charges were immediately filed against the man, who hasn't been identified by police.
Chicago Transit Authority security cameras caught the man on tape before the shooting, but McCarthy didn't say if the shooting was captured by cameras.
The incident left passengers shaken, with some refusing to get off when the doors opened.
"There were four or five shots (that) hit the train — boom, boom, boom," said John Langston, 50, of Chicago. "I thought it was somebody with a sledgehammer."
Passenger Venesia Parker said that when the train stopped, "Some rushed out, but I stayed on for safety until I could see nobody was out there."
Both she and Langston saw shell casings as they walked through the station near the Chicago Board of Trade building in the city's financial district.