The 60-year-old Nivakoff says his health was one of the major factors behind his decision to step down at the end of this month.
He says he has turned over many of his duties to Assistant Chief John Fontneau and is recommending him as his successor.
Both Nivakoff and Fontneau were off duty Saturday and did not immediately return calls to their homes for comment.
Nivakoff is credited with leading efforts to quickly transform the Stamford police department, including bringing the SWAT team, a bomb squad, special dog units and hostage negotiation skills.
Nivakoff publicly announced his upcoming retirement on Thursday.
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