DATELINE: DALLAS — Feds have asked a US judge to decide who NOW owns a handwritten letter that JACKIE KENNEDY wrote to BOBBY KENNEDY’s widow ETHEL after the RFK assassination 1968.

According to the FBI’s paper trial, a plumber stole the letter from Ethel Kennedy's home.

After the plumber died, his son, seemingly unaware of how his father acquired the heart-rending missive, then sold the letter to an archivist who later resold it.

The so-called “Jackie letter” was then sold and resold, for higher and higher prices, until Robert and Ethel Kennedy's son, Max, learned of it.

AS The ENQUIRER reported previously, he then asked the Feds to seize it in an attempt to establish true ownership.

Federal agents holding the bombshell letter in, ironically, the FBI Dallas field office, want the judge to decide whether the letter belongs to the Kennedy heirs or to the last person who had possession, Richard Goodkin.

“Jackie’s letter” was first sold for $25,000 in May 2001 to a New Hampshire collector who traded it in February 2004 for a letter written by Martha Washington AND a letter signed by President William Henry Harrison — THAT collector sold it in August 2004 to Goodkin "as repayment for a debt."

Feds say the letter was valued as $25,000 to $30,000 when Goodkin consigned the letter in July 2006 to Heritage Auction Galleries in Dallas. Two months later, Max Kennedy, son of Ethel and Robert Kennedy, notified the FBI that Heritage had the alleged stolen letter.

"(Max) Kennedy advised the FBI that the letter was the personal property of his mother, Ethel Kennedy, and that neither she nor anyone else in the Kennedy family had given permission to be removed from the Kennedy home, or further sold, traded, or otherwise transferred," according to the complaint.

The FBI seized the letter with a search warrant in August 2009, "as part of a criminal investigation … prohibiting the sale or receipt of stolen goods had occurred. That investigation is now concluded and no prosecution is expected."

The FBI says it's holding onto the letter due it its "personal nature and historical significance" until a judge rends final decision.