A mysterious two page handwritten letter from JFK’s widow Jackie to brother-in-law Robert F. Kennedy‘s widow has been seized by the FBI.

"There’s still an ongoing investigation," said FBI spokesman Mark White.

At issue is the ownership of the letter as it went from the Virginia home of Ethel Kennedy, RFK’s widow, to an locked evidence vault in Dallas FBI field office was detailed in six-page affidavit filed last month in U.S. District Court in the Northern District of Texas by Special Agent John Skillestad.

Max Kennedy, RFK’s son, said he alerted the Feds after the missive found itself on the auction block in July 2006 with prestigious Heritage Galleries and Auctioneers in Dallas.

Max "stated that he had not given authority to sell, give, or donate any papers of Ethel or Robert Kennedy to anyone," the affidavit says.

Last October agents spoke with Ethel Kennedy, 81, who said she "never gave the letter to anyone to keep and under no circumstances would anyone have the authority to transfer ownership of the letter to anyone."

The FBI affidavit states the Jackie O letter "is considered as a stolen good."

Jackie’s letter is not dated but is believed to have been penned shortly after the assassination of RFK on June 6, 1968 to grieving widow Ethel.

The ENQUIRER previously reported that Jackie and Bobby had an alleged four-year affair after the assassination of JFK in 1963, according to new book Bobby and Jackie: A Love Story.