Just cut this from the wiki article about Jack Ruby;
'After his arrest, Ruby asked Dallas attorney Tom Howard to represent him. Howard accepted and asked Ruby if he could think of anything that might damage his defense. Ruby responded that there would be a problem if a man by the name of "Davis" should come up. Ruby told his attorney that he "... had been involved with Davis, who was a gunrunner entangled in anti-Castro efforts".: 44 
Later, Ruby replaced attorney Tom Howard with prominent San Francisco defense attorney Melvin Belli, who agreed to represent Ruby pro bono. On March 14, 1964, Ruby was convicted of murder with malice and was sentenced to death.
Ruby's conviction was overturned by the Texas Court of Criminal Appeals on the grounds that "an oral confession of premeditation made while in police custody" should have been ruled inadmissible, because it violated a Texas criminal statute. The court also ruled that the venue should have been changed to a Texas county other than the one in which the high-profile crime had been committed. Ruby died technically unconvicted, because his original conviction was overturned and his retrial was pending at the time of his death.
During the six months following Kennedy's assassination, Ruby repeatedly asked, orally and in writing, to speak to the members of the Warren Commission. The commission initially showed no interest. Only after Ruby's sister Eileen wrote letters to the commission (and her letters became public) did the Warren Commission agree to talk to Ruby. In June 1964, Chief Justice Earl Warren, then-Representative Gerald R. Ford of Michigan, and other commission members went to Dallas to see Ruby. Ruby asked Warren several times to take him to Washington D.C., saying "my life is in danger here" and that he wanted an opportunity to make additional statements.: 194–196 He added: "I want to tell the truth, and I can't tell it here.": 194 Warren told Ruby that he would be unable to comply, because many legal barriers would need to be overcome, and public interest in the situation would be too heavy. Warren also told Ruby that the commission would have no way of protecting him, since it had no police powers. Ruby said he wanted to convince President Lyndon Johnson that he was not part of any conspiracy to kill Kennedy.: 209–212
Eventually, the appellate court agreed with Ruby's lawyers that he should be granted a new trial. On October 5, 1966, the court ruled that his motion for a change of venue before the original trial court should have been granted. Ruby's conviction and death sentence were overturned. Arrangements were underway for a new trial to be held in February 1967 in Wichita Falls, Texas, when on December 9, 1966, Ruby was admitted to Parkland Hospital in Dallas, suffering from pneumonia. A day later, doctors discovered cancer in Ruby's liver, lungs, and brain. His condition rapidly deteriorated'
The action of the Warren Commission does raise the question, why didn't they want to hear his testimony?.