Wikipedia's entry on Jack Ruby

To briefly summarise, he was arrested immediately after he killed Oswald and convicted of murder on March 14th 1964, when he received the death sentence. Whilst in jail, he begged Chief Justice Earl Warren to take him to Washington DC so he could testify as to his involvement in the assassination, and also because he feared for his life. Warren denied his request and Ruby, in spite of being granted a new trial a few years later, died before it could take place of a pulmonary embolism on January 3rd 1967