"We have not been able to determine exactly what happened or who was behind the breach but we do believe that the digital domain still is the main suspect," the permanent secretary of the committee that bestows the award told Reuters on Friday.
Bets on Venezuelan opposition leader Maria Corina Machado winning the prize spiked on October 10, hours before the name of the 2025 laureate was due to be announced in Oslo.
An initial bet was placed on Machado's name to win the prize, rising quickly to $US2.2 million ($A3.1 million) as others followed, Kristian Berg Harpviken said.
Machado's name had not been mentioned ahead of time by any of the experts nor the media.
One of Norway's three intelligence agencies was involved in the ensuing investigation of whether there was an internal leak or whether it was the result of spying, either by a criminal actor or a state entity, but it remains unclear who was behind the leak and how it unfolded, Harpviken said.
And while financial bets were placed, it was not known whether the ultimate purpose was to profit from the incident or to inflict damage to the credibility of the prize, he said, adding that the institute's efforts have now turned to preventing future breaches.
Machado won the award for fighting dictatorship in Venezuela and dedicated the award in part to US President Donald Trump, who has repeatedly said he deserved to win himself.
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