Cause of death: cancer
Jack Leon Ruby (born Jacob Leon Rubenstein; c. March 25, 1911 – January 3, 1967) was an American nightclub owner who killed Lee Harvey Oswald on November 24, 1963, two days after Oswald was accused of assassinating President John F. Kennedy. Ruby shot and mortally wounded Oswald on live television in the basement of Dallas Police Headquarters and was immediately arrested.
In a trial, Ruby was found guilty and sentenced to death. Ruby's conviction and death sentence were overturned on appeal, and he was granted a new trial, but he became ill, was diagnosed with cancer, and died of a pulmonary embolism on January 3, 1967.
In 1964, the Warren Commission concluded that Ruby acted alone in killing Oswald and that Ruby shot Oswald on impulse and in retaliation for the Kennedy assassination. The commission's findings would be challenged by various critics who hypothesize that Ruby was part of a conspiracy surrounding the Kennedy assassination.
Source : Wikipedia