Brady was shot in the head and partly paralyzed, spending the rest of his life in a wheelchair and advocating against gun violence.
His family said in a statement that he died from health issues at 73. But a Virginia medical examiner has ruled the case a homicide, prompting a new investigation, Metropolitan Police spokeswoman Gwen Crump said.
Bill Miller, a spokesman for the U.S. attorney in Washington, said his office is reviewing the medical examiner's findings. He had no further comment.
John Hinckley, the lone gunman who fired the shots that wounded Reagan, Brady, a police officer and a Secret Service agent outside a Washington hotel, was found not guilty by reason of insanity.
He has spent the ensuing years in a psychiatric hospital.
Hinckley was charged at the time with assault with intent to kill and assault with a dangerous weapon. It's unclear if he will face any new counts.
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