LONDON — Lost documents said to be a “conspiracy theorist’s dream come true” have been unearthed which suggest that Lee Harvey Oswald and Jack Ruby plotted together to kill President Kennedy.

According to the Warren Commission, the official inquiry into the assassination, Oswald acted alone when he shot Kennedy as the president’s motorcade swept past the Texas School Book Depository in Dallas. Two days later, Ruby shot Oswald dead as police were escorting him for questioning.

The boxes of evidence, found in an old safe in a Dallas courthouse, include a transcript of a conversation said to be between the pair discussing how they would carry out the assassination on behalf of the Mafia.

The transcript claims that the real target of the Mob was the president’s brother, Robert Kennedy, the attorney general, himself later assassinated. Robert had launched a campaign against the criminal underworld. In the alleged conversation, Ruby, a nightclub owner, and Oswald decide that it would be easier to kill the president than his brother and that JFK’s death would end the inquiry just as effectively.

The Dallas County district attorney, Craig Watkins, and investigators discovered the “treasure trove” on the 10th floor of their office after rumors circulated that the gun Ruby used to kill Oswald was hidden there.

They never found the weapon but discovered the dozen boxes in a safe which are believed to refer to Ruby’s trial. He was convicted of killing Oswald but the verdict was overturned. He died of cancer before a planned retrial.

The haul includes personal letters to and from the then district attorney, Henry Wade, a gun holster, and clothes that probably belonged to Ruby and Oswald.

But most attention has been focused on the transcript of a conversation said to be between Ruby and Oswald at the former’s nightclub on October 4, 1963, two months before the assassination on November 22.

In it, they talked of killing the president because the Mafia wanted to “get rid of” his brother. Oswald is recorded as saying: “I can still do it, all I need is my rifle and a tall building; but it will take time, maybe six months to find the right place; but I’ll have to have some money to live on while I do the planning.” While Mr. Watkins believes the documents will re-open the debate, the evidence has been challenged by experts.

They believe the documents are more likely to be rough drafts of a documentary-style film that Wade was said to be researching but never made.

“It will open the debate again and whether there is a conspiracy,” Mr. Watkins said. “You know me: I’m always a conspiracy theorist.”

But the curator of the Sixth Floor Museum in Dallas near where the president was shot, Gary Mack, has doubts.

“It is well-documented that Oswald was in Irving, Texas, on the evening of October 4, at a home where his wife was staying,” he said. “The fact that this paper is sitting in Henry Wade’s file, and he didn’t do anything, indicates he thought it wasn’t worth anything.” The former prosecutor wrote about the proposed film, “Countdown in Dallas,” in other letters found in the safe. The transcript of the conversation resembles one published by the Warren Commission.