Background: Reasoner was born at First Street North in Dakota City, Iowa; he and his older sister, Esther, were the children of Eunice (Nicholl) and Harry Ray Reasoner, who married in 1911. Reasoner was taught to read by his parents before entering school, gaining a strong vocabulary from his mother. Reasoner attended West High School in Minneapolis. During his time at the school, Reasoner developed his interest in journalism.
Context: Reasoner took part in covering the John F. Kennedy assassination on Friday, November 22, 1963. Walter Cronkite and Charles Collingwood had been switching back and forth to report on the incident for about four hours after Cronkite initially broke the news at 1:40 p.m. EST. Reasoner took over the anchor chair after Collingwood tossed it to him at 5:49 p.m. EST and opened with the repeat of an announcement by Frank Stanton, the president of CBS, which had already been relayed by Collingwood:  He later reported on the arrival of Kennedy's body in Washington, D.C., and provided details regarding Lee Harvey Oswald (who was then accused only of killing Dallas Police Officer J. D. Tippit; he would be accused of killing the president only hours later). Reasoner left the desk when Cronkite returned to anchor CBS Evening News at 6:35 p.m. EST. He reappeared in another studio, approximately two hours later, to narrate a special program called "John F. Kennedy -- A Man of This Century", and he talked about Kennedy's career and the new president, Lyndon B. Johnson and announced the conclusion of CBS's coverage for that day. (Reasoner also anchored the final coverage of the next day, with a CBS News special titled "A Day to Mourn".)  Reasoner's next appearance came two days later, Sunday, and as Reasoner was at the anchor desk, Oswald was shot by Jack Ruby while being moved in the Dallas City Jail. At that very moment, Roger Mudd was filing a report from Washington, describing the President's funeral arrangements.  At this point, CBS abruptly cut back to Reasoner at the newsroom anchor desk with breaking news.  At the time CBS picked up KRLD's live feed of the city jail basement, Oswald was lying on the floor and Dallas Police were apprehending Ruby. KRLD's reporter on the scene, Bob Huffaker, was heard to say, "Oswald has been shot, Oswald has been shot". After the ambulance carrying Oswald sped out of the jail en route to Parkland Hospital, KRLD switched back to CBS in New York, where Reasoner replayed the tape from the beginning so the viewers could see Ruby shooting Oswald. Several minutes later, he reported that the Dallas Police had released Ruby's name. (Reasoner was not at the anchor desk when Oswald was declared dead since he had been replaced by Cronkite.)
Question: what was his biggest contribution to the assassination

Answer:
Reasoner's next appearance came two days later, Sunday, and as Reasoner was at the anchor desk, Oswald was shot by Jack Ruby while being moved in the Dallas City Jail.