Hosty died June 10 of cancer at a hospice in Kansas City, Mo.
"He was a man on a mission," Hosty's son Tom told the Kansas City Star. "He was determined to get the entire story out there to the American public — to set the record straight."
As recently as 2003, James Hosty told the Star there was nothing he could have done to prevent the assassination given what he knew at the time. He also conceded that he probably would "go to my grave trying to straighten this out."
Long before Kennedy's assassination, Oswald was well-known to the FBI and the Central Intelligence Agency. A former Marine, Oswald had defected to the Soviet Union in 1959, a move that made international front-page news.
Hosty said that in September 1962, after Oswald returned to the U.S., the FBI agent who had the Oswald file determined that it should be officially closed. When that agent retired a month later, Hosty inherited his files. But in early 1963, Hosty came to believe that Oswald and his wife needed further investigation.
He suspected Oswald's wife, Marina, might be a Soviet "sleeper" agent who married him only to enter and spy on America.
He also noticed that right after Oswald disavowed the Soviet Union and the FBI closed its file on him, Oswald bought a subscription to the Daily Worker, the U.S. Communist Party newspaper. The FBI reopened the Oswald file.
"I'm sorry I ever got the case," Hosty said in the 2003 interview.
He said the case was never considered a priority, and the bureau believed that if the Oswalds were involved in anything, it was probably no more than low-level espionage.
After Kennedy's assassination, Hosty was appointed to help lead the FBI's investigation. The Warren Commission Report noted Hosty by name and implied that he had been negligent.
"He always carried it with him," Tom Hosty said.
Hosty was born Aug. 28, 1924, in Chicago. He served in the Army during World War II, graduated from the University of Notre Dame in 1948 and joined the FBI in 1952.
He was later transferred from Dallas to Kansas City and retired from the bureau in 1979.
Hosty and his wife, Janet, had nine children. He is survived by seven children, 22 grandchildren, nine great-grandchildren and four sisters.