Witnesses told Channel 2 they heard four rounds coming from the loading dock of the post office around the time the woman was shot in the head.
A witness also said he saw the shooter sitting at a Coin Laundry just before the shooting.
Police hope cameras from that laundromat can help them identify the shooter.
Postal inspectors are investigating the shooting and working to gather details, USPS spokesman Rick Badie told The Atlanta Journal-Constitution.
No other details have been released.
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— Staff writer Lauren Foreman contributed to this article.
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