Capt. Gerald McKinney told The Providence Journal Thursday he signed retirement papers this week, but declined to say why.
McKinney filed a complaint with the state's human rights commission, alleging that superintendent Col. James Manni and others "engaged in discriminatory practices." McKinney is black.
When asked about McKinney's retirement Thursday, Manni told the newspaper he's prohibited from speaking about internal investigations.
McKinney's treatment of a 17-year-old robbery suspect in 2014 is the subject of an internal inquiry.
Manni asked the Connecticut State Police to review whether the incident was handled properly and thoroughly investigated. The results were recently forwarded to prosecutors to review.
McKinney says he plans to move his complaint forward, possibly into federal court.
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