According to the Warren Commission Exhibit 832, Texas School Book Deposistory employee Lee Oswald "claimed to be on the first floor when President John F. Kennedy passed" the building and "frantically denied shooting Dallas police officer Tippett or shooting President John F. Kennedy."
But the FBI had made a special point of learning where Oswald worked only a few weeks before JFK was ambushed. According to Warren Commission Exhibit 837, an April 6, 1964 memorandum from J. Edgar Hoover to the Warren Commission, after the roommate of Oswald's wife, Mrs. Ruth Paine, "advised on Nov. 1, 1963, of Oswald's employment at the Texas School Book Depository, a pretext telephone call was made to the Texas School Book Depository to determine Oswald's residence address" and "advice was received that Oswald was working at Texas School Book Depository..."
Coincidentally, after JFK was ambushed near the Texas School Book Depository on Nov. 22, 1963, the FBI blamed the assassination on the Texas School Book Depository employee who had been under its surveillance.