Three months ago, Dexter Scott King declared that he and his family believed that James Earl Ray was not guilty of the murder of his father, the Rev. Dr. Martin Luther King Jr. Tonight, in a televised interview, Mr. King asserted that President Lyndon B. Johnson must have been part of a military and governmental conspiracy to kill Dr. King.

''Based on the evidence that I've been shown, I would think that it would be very difficult for something of that magnitude to occur on his watch and he not be privy to it,'' Mr. King said on the ABC News program ''Turning Point.''

Mr. King, who heads the Martin Luther King Jr. Center for Nonviolent Social Change in Atlanta, suggested that the Army and Federal intelligence agencies were involved in his father's assassination, in Memphis on April 4, 1968.

''I am told that it was part and parcel Army intelligence, C.I.A., F.B.I.,'' he said in the interview. ''I think we knew it all along.''