Who killed Officer Tippit?

[Major Nikon]

4. He was out of his beat area because he was directed to do so

Approximately 14 minutes later, and just 45 minutes after the assassination, another violent shooting occurred in Dallas. The victim was Patrolman J. D. Tippit of the Dallas police, an officer with a good record during his more than 11 years with the police force. He was shot near the intersection of 10th Street and Patton Avenue, about nine-tenths of a mile from Oswald's roominghouse. At the time of the assassination, Tippit was alone in his patrol car, the routine practice for most police patrol officers at this time of day. [div style="display:inline; background-color:#FFFF66;"]He had been ordered by radio at 12:45 p.m. to proceed to the central Oak Cliff area as part of a concentration of patrol car activity around the center of the city following the assassination.

At 12:54 Tippit radioed that he had moved as directed and would be available for any emergency. By this time the police radio had broadcast several messages alerting the police to the suspect described by Brennan at the scene of the assassination -- slender white male, about 30 years old, 5 feet 10 inches and weighing about 165 pounds.

http://www.archives.gov/research/jfk/warren-commission-report/chapter-1.html