Hoax Threat May Have Led to FBI Sony Alert Tweeter says he was "messing around" with threat to news media.

 -- Federal authorities are trying to determine whether a threat to news media that covered the Sony hack was a hoax by a tweeter who says he was just "messing around."

The FBI and the Homeland Security issued a joint intelligence bulletin last week to law enforcement agencies across the country.

The bulletin lays out much of what the public already knows: threats against Sony in late November if it released "The Interview," the massive cyber-hack of Sony days later, the subsequent threat of a 9/11-style attack on theaters showing the film, and then Sony's hesitation in releasing the film.

But the bulletin also discloses that threats -- ascribed to the alleged Sony hackers known as "Guardians of Peace" -- have "extended to … a news media organization … and may extend to other such organizations in the near future."

Hours after news surfaced of the FBI and DHS bulletin, a man claiming to be David Garrett Jr. in Tennessee said on Twitter that he posted the "fake" threat against the news organization, but he was just "messing around."

The FBI said it was looking into the claim, which ABC News has been unable to independently verify.

Asked why the FBI would issue a bulletin based on uncorroborated information, a spokesman said it comes down to taking precautions.

"As part of our commitment to public safety, the FBI routinely shares information with the private sector and law enforcement community," the spokesman said in a statement. "We take all threats seriously and will continue to disseminate relevant information observed during the course of our investigations, in order to help protect the public against any potential threats."

On Dec. 20, "Guardians of Peace" posted messages online "that specifically taunted the FBI and [the unidentified news organization] for the 'quality' of their investigations and implied an additional threat," reads the bulletin.

While the bulletin says there is no "specific credible information" to indicate any type of "physical threat," and notes "that hacking groups have historically made exaggerated threat statements," the bulletin urges law enforcement "to remain vigilant."

"[T]he potential remains for [Guardians of Peace] or copycat actors to make renewed cyber and/or implied physical threats, to identify new targets, or execute physical attacks if the movie is again scheduled for release," according to the bulletin, first reported by the website The Intercept and confirmed by ABC News.

In addition, the bulletin predicts "Guardians of Peace" will "likely continue to release portions of proprietary data and email stolen from [Sony] to keep pressure on [the company], possibly to get the company to withhold further distribution of the movie, destroy all copies of the film or publicly apologize for its production of 'The Interview,'" which depicts the fictional assassination of North Korean leader Kim Jong Un.

In the face of skeptics increasingly voicing their own theories about who is responsible for the Sony hack, the FBI is firmly standing by its announcement two weeks ago blaming North Korea.

"There is no credible information to indicate" otherwise, and the FBI's conclusion is based on intelligence gathered by the U.S. government and foreign governments, according to an FBI spokesman. Anyone without the proper security clearances would not have access to that intelligence.

As ABC News reported two weeks ago, federal authorities believe an individual or small group stationed outside North Korea – but acting at the behest of the North Korean regime – may have punched the computer keys that launched the attack.

The North Korean government has denied any role in the cyber-attack on Sony, calling it "wild speculation." Still, the North Korean government has described the attack as a "righteous deed."

ABC News' Pierre Thomas contributed to this report.