By Dana Kozlov

CHICAGO (CBS) — Oxygen tanks must be on every single Chicago ambulance.

But CBS 2 Political Investigator Dana Kozlov learned there was a delay with the tanks’ delivery for Fire Department ambulances just this week. And that could have meant a disaster if supplies had run out.

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Illinois state law requires the oxygen tanks in all ambulances. But sources said this week, the tank delivery was delayed by days.

Instead of arriving at the beginning of the week, Chicago Fire Department spokesman Larry Langford said the latest oxygen tanks supply was being delivered Friday.

That is at least three days after the tanks should have arrived.

Why does it matter? Sources said Chicago Fire Department paramedics and firefighters were concerned that oxygen tank supplies could run short – or even run out.

Some ambulances also had to be driven greater distances to restock.

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Both issues could have taken ambulances out of service. That is a concern, considering CBS 2 Investigator Pam Zekman has exposed an ambulance shortage in the city – one that Mayor Lori Lightfoot acknowledged just on Wednesday.

“We know that we need more ambulances, and it’s my expectation, when we finalize a new fire contract, there will be more ambulances coming on line,” Lightfoot said Wednesday.

Langford blamed the delivery delay on a contract glitch.

A source said it was because the CFD didn’t renew its contract with the oxygen tank supplier. The CFD could not confirm that, referring us instead to the city’s Procurement Office.

A spokeswoman for that office said the contract with the oxygen supplier lapsed on Dec. 3, and the company did not sign an extension – hence the delay.

She said a new contract with a new company is now in place.

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Meanwhile, we are told no ambulances were ever actually taken out of service in connection with the oxygen delivery delay.