The incident took place near Oakley and Armitage avenues, and was captured on surveillance video.
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The homeowner whose camera captured the whole crime did not want to be identified, but described what the video shows.
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"It was just shocking. He delivered our mail and turned around, and he was right there at the fence holding him up," she said. "It was very scary for me watch it and for him to experience that. I feel very bad."
The president of the Letter Carriers Union said since mid-August more than 20 mail carriers have been robbed across the Chicago area.
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The growing crime trend is causing growing concern.
"They have families at home. They want to get home to their family at the end of the day, too," Mack Julion said. "One letter carrier told me, she called today and said 'this is not what I signed up for.' She said, 'I'm quitting this. I did not sign up for people jumping out of cars, pulling a gun on me.'"
Another mail carrier was also robbed on Tuesday in Oak Park.
The U.S. Postal Inspection Service is working with police to investigate.