CIA source states CIA was involved in JFK Assassination - why are thousands of docs still hidden?

Documents about the JFK assassination has been kept sealed for nearly 60 years. Why? Why are some still hidden? A CIA Source may have just stated part of the answer. If the CIA was involved in killing a sitting U.S. President, what else might they have been involved in? Why? The term "conspiracy theory" only came into American language beginning in 1964 based on stories in the New York Times - coincidence?