Yesterday the FBI announced that the North Koreans were behind the Sony hack.

Now the North Koreans are denying it, via BBC:

North Korea has offered to hold a joint inquiry with the United States into a cyber-attack on Sony Pictures, strongly denying US claims that it is behind it.

Its foreign ministry accused the US of “spreading groundless allegations”, which the joint probe would refute.

Without addressing Pyongyang’s idea, a US spokesman insisted that North Korea must admit “culpability” …

On Saturday, the North Korean foreign ministry said: “As the United States is spreading groundless allegations and slandering us, we propose a joint investigation with it into this incident.”

“Without resorting to such tortures as were used by the US CIA, we have means to prove that this incident has nothing to do with us.”

The statement said there would be “grave consequences” if the Americans rejected their inquiry proposal.

If not North Korea, then who? Here are four possibilities via NY Mag:

IT WAS A DISGRUNTLED SONY INSIDER, OUT FOR REVENGE

Marc W. Rogers, a “whitehat” hacker and security researcher at the online-traffic-optimizer CloudFlare, wrote on his personal blog that he’s betting on “a disgruntled (possibly ex) employee of Sony.” He notes that there were multiple ways hackers could have made money from stealing Sony’s information, and a nation-state could have used its access to Sony to gain more information about the film industry. Instead, Guardians of Peace chose to dump the information in a manner that’s extremely embarrassing to Sony.

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IT WAS A GROUP OF HACKTIVISTS

In its excellent examination of the evidence in the Sony hack, Wired concludes that it’s more likely that the breach was the work of hacktivists such as Anonymous or LulzSec, rather than a nation-state. The FBI said it found “links to other malware that the FBI knows North Korean actors previously developed,” but hackers sophisticated enough to get into Sony’s systems would presumably also have the ability to plant false clues in their coding.

* * *

IT WAS THE CHINESE

The U.S. has already had multiple run-ins with hackers in the Chinese military, and last year it was reported that they were targeting various American companies. A U.S. official told Reuters on Friday that while North Korea was behind the operation, there was a link to China, either through collaboration with Chinese hackers or using Chinese servers to mask the hack’s origin. Statements from U.S. officials today did not mention China.

The cybersecurity firm Mandiant has been brought in to investigate the Sony hack, and according to Deadline, that could be another sign that the Chinese government was involved. “Mandiant has investigated so many Chinese attacks,” said a source. “It’s kind of their forte.”

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EVERYBODY DID IT

There’s overlap in many of these theories, and it’s entirely possible that the answer is “all of the above.” Many have said Sony was lax about its cybersecurity, and as Wired notes, “we can’t rule out the possibility that nation-state attackers were also in Sony’s network or that a nation like North Korea was supportive of some of these hackers, since they shared similar anger over Sony.”