Some residents walked around carrying baseball bats, just in case.
Hours later, the big cat emerged from underneath a porch. Officers chased it down an alley and eventually opened fire, killing it.
"The animal's very excited, it's disturbed, it's scared. It could have attacked a person if it was cornered," said Cmdr. John Kenny of the Chicago Police Department.
Officers said there was no time to find a tranquilizer gun. It was unclear where the 150-pound cougar came from, but they don't think it's the same one that was seen in the northern suburbs over the weekend.