Richard A. Brown, the Queens County district attorney who in almost three decades in that office prosecuted police officers accused of committing unjustified killings, robbery defendants who executed potential witnesses and a doctor convicted of murder for fatally botching an abortion, died on Saturday in a hospice care facility in Redding, Conn. He was 86.

The cause was complications of Parkinson’s disease, his son, Todd Brown, said.

In January, after seven terms in office, Mr. Brown announced that he would not seek re-election. In March, with his seventh term in office scheduled to end on Dec. 31, Mr. Brown announced that he would step down on June 1, “the 28th anniversary of my first assuming this office.”

He said he had hoped to finish out the term, but given his health issues, it had become “increasingly difficult to fully perform the powers and duties of my office in the manner in which I have done since 1991.”

Mr. Brown, a former judge who left the calm of an appellate court for the pressures of a big-city prosecutor’s office, was known in his early years on the job for showing up at crime scenes, an unusual practice for the city’s district attorneys.