Officials told local media a fire crew found the boy in the snow-covered bag around 5am on Saturday outside the Near North Side fire station.

"They were in and out so much that morning that no one heard the doorbell," Chicago Fire Department spokesman Larry Langford said.

Baby Safe Haven Law

In the state of Illinois parents can hand their newborn over at hospitals, police or fire stations without fear of prosecution under the state's Baby Safe Haven Law.

The firehouse was a designated safe haven.

Authorities have said they do not know how long the baby was left outside in freezing temperatures, or whether he was alive when he was left.

Baby should have been handed over

Mr Langford said the fire station is used as an air supply maintenance facility, meaning crews are often out servicing supplies at other firehouses.

He said though the firehouse was a safe haven, babies must be "passed person-to-person".

"We're trying to make it clear that you have to make contact. Ringing a bell is not making contact. You have to physically see someone and hand the child over," Mr Langford said.

"If the person left the child there, and we don't recommend it, and called 911 to say the child is there, somebody would've been dispatched right there."

The police are investigating and are working to obtain surveillance footage.