The checks were being mailed to vendors in payment for campaign services, WFLD-TV, Chicago, reported.
They were taken from their envelopes before they were mailed, campaign workers told police.
Two of the checks were written for more than $20,000 each. At least one of the checks had already been deposited into a downtown bank account.
Security is tight inside the Prudential Plaza complex where the campaign offices are located. Anyone going to the campaign offices must be credentialed to leave the lobby.
"Three checks we sent to vendors made it into the wrong hands, and after learning of this, we notified the police. We're pleased that they're dealing with the issue swiftly, and don't expect this to have any impact upon the campaign," Ben Finkenbinder, Obama's Midwest press secretary, said in a statement Wednesday, the Chicago Sun-Times reported.
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