The van was stolen from the 2500-block of North Ashland Avenue around 10:15 a.m., a U.S. Postal Inspector spokeswoman said.
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The mail carrier was not harmed, but the van with its letters and packages was still missing Tuesday afternoon.
The spokeswoman said it's standard procedure for mail trucks to be turned off while carriers are making deliveries, but she declined to confirm if that rule was followed Tuesday.
The truck originated from the 2600-block of North Clark Street, where there is a USPS office.
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Chicago police are assisting, but the U.S. Postal police are leading the investigation.
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It was not clear Tuesday how the post office would notify customers if the van's contents are not recovered.
Anyone who notices suspicious activity involving a mail truck is asked to call Chicago police.