From the Annals of Police Incompetence –  On November 24, Jack Ruby drove to downtown Dallas  with his two pet dogs and sent an emergency money order to one of his employees, Ruby then walked to the nearby police headquarters, where he made his way to the basement via the Main Steet ramp. Shortly before noon, officers from the Dallas Police Department were escorting Lee Harvey Oswald through the police basement to an armored car that was to take him to the nearby county jail.  Ruby emerged from the crowd of reporters and fired his .38 revolver fatally wounding Oswald. The shooting was broadcast live nationally, and millions of television viewers witnessed it.

The 1979 report from the House Select Committee on the Assassination stated:

Ruby’s shooting of Oswald was not a spontaneous act, in that it involved at least some premeditation. Similarly, the committee believed it was less likely that Ruby entered the police basement without assistance, even though the assistance may have been provided with no knowledge of Ruby’s intentions… The committee was troubled by the apparently unlocked doors along the stairway route and the removal of security guards from the area of the garage nearest the stairway shortly before the shooting… There is also evidence that the Dallas Police Department withheld relevant information from the Warren Commission concerning Ruby’s entry to the scene of the Oswald transfer. 

Well, duh.