Capitol police spokeswoman Sgt. Kimberly Schneider confirmed they were investigating the incident but declined to discuss details.
The Hill newspaper's Web site and Atlanta television said the Georgia congresswoman hit an officer who asked for her identification when she stepped round a metal detector.
Members of Congress are allowed to walk past the detectors, set up for staff and members of the public. But it was unclear whether McKinney was wearing a congressional identification pin or whether the officer had been able to see it.
An aide in McKinney's office declined immediate comment.
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