The three-year-old alligator was later transferred into the care of the Chicago Herpetological Society, the report said.
The American alligator was still "in distress" from the incident and was suffering from a metabolic bone deficiency, a spokesman for the society said.
A passenger going down an escalator apparently spotted the alligator first and he grabbed a custodian who then called police, the spokesman said.
Allie was likely "dumped" there by an unknown person who carried the alligator to the airport, he said.
"Some human being physically carried it there and put it there," the spokesman said.