The medical examiner ruled Friday that Brady's death Monday at the age of 73 was a homicide, WRC-TV reported.
The finding could allow federal prosecutors to seek to try John Hinckley Jr. for murder. Hinckley has been confined to St. Elizabeth's Hospital in Washington since he was found not guilty by reason of insanity of wounding Brady, Reagan, a Secret Service agent and a police officer on March 30, 1981.
Brady, the most seriously injured, was partially paralyzed for the rest of his life. The medical examiner found that his injuries eventually killed him.
Hinckley ambushed Reagan as the president walked back to the White House after giving a luncheon speech at the Washington Hilton. He was apparently driven by an obsession with the actress Jodie Foster and with one of her early movies, "Taxi Driver," which starred Robert De Niro as an obsessed New York cabbie.
Brady and his wife, Sarah, became the country's best-known advocates for gun control after the shooting. The 1993 law requiring background checks for gun owners was named in his honor.