"Our motto has always been that everything is open. We have nothing to hide. So we're making public everything that we have found in the safe, adding that we have learned about it shortly after taking office in 2007.
"Every DA to my knowledge has been made aware of the contents of that safe and every DA, until this new administration, decided that it wanted to keep it secret for whatever reason," Watkins told a press conference in Dallas
"We decided that this information was too important to keep.
The courthouse overlooks the site where on November 22, 1963, hit by bullets as he passed in an open top car with his wife Jacqueline
The prosecutor said the discovery could fuel new conspiracy theories but one of the most intriguing documents has already been discounted as a likely fraud.
It is a purported transcript of a conversation between killer, Lee Harvey Oswald, and Jack Ruby, the club owner who killed Oswald before he could face trial.
Watkins said he didn't know whether or not the conversation ever took place.
"But what we do know is that it will open up the debate as to whether or not there was a conspiracy to assassinate the president," he said.
In the alleged conversation, the two men discuss a plot to kill a month before the popular assassination in Dallas, according to US media.
Terri Moore, Watkins' top assistant, was quoted as saying in the Dallas Morning News that she believes the transcript is part of a movie the district attorney at the time of the assassination was working on with producers.
"It's not real. Crooks don't talk like that," she was quoted as saying by the Morning News. "If that transcript is true, then history is changed because Oswald and Ruby were talking about the president."
The document stash reportedly includes a proposal for a movie signed by then-district attorney Henry Wade.
Other discoveries include Oswald's clothing, Ruby's leather holster and brass knuckles that Ruby carried when he killed Oswald.
There are also official records from the trial of Ruby, who saw a verdict against him overturned and died of cancer in 1967 before his second trial could begin.
According to excerpts of the suspect text, published by the newspaper, Oswald discusses a plot to kill the attorney general, the councelor.