By Dana Kozlov

CHICAGO (CBS) — Thieves have been using master keys to steal mail and police have been tracking them for months – yet it’s still happening.

But how are the thieves getting a hold of the master keys? A former post office manager told CBS 2 Political Investigator Dana Kozlov Tuesday why the system to track them does not always work.

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“You have to sign in for those keys, and the clerk clears you at the end of the day when you return,” said the former supervisor.

That is the way it is supposed to work inside the United States Post Office. The former supervisor, who did not want to be identified, said carriers get assigned several sensitive items before they start their routes.

One is a master key – also called an arrow key – that opens all locked mailboxes, like the ones inside multi-unit buildings. They are supposed to be logged and returned at the end of a carrier’s shift.

“You had to be checked,” the former supervisor said. “You could not leave unless we checked everything.”

But as we have been reporting, thieves have been using those keys to access mailboxes and steal mail. Most recently, two suspects were caught on security camera accessing and rifling through mail in a condo building in the Clybourn Corridor area.

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Even though there is a system in place to track those master or arrow keys, our former supervisor acknowledges it is not foolproof.

“I’ve had them lost, stolen, misplaced,” he said. “There’s some keys that have been missing for a few years.”

And he said once a master key is gone, it’s gone.

So we submitted a public records request, asking the USPS how many thefts involving master keys are currently being investigated. The answer is 41 – but they couldn’t say how many of those cases specifically involve the use of a master key.

And oddly, the investigation into the thefts is not on the list of open investigations – even though it was reported this month, and a spokesperson told us postal inspectors are working on it.

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We will once again follow up.