The Sony hackers made a big mistake by logging into Facebook, according to a report in the New York Times. The mistake, according to the report, revealed the hackers were working for North Korea.

“[S]enior government officials said that F.B.I. analysts discovered that the hackers made a critical error by logging into both their Facebook account and Sony’s servers from North Korean Internet addresses. It was clear, the officials said, that hackers quickly recognized their mistake. In several cases, after mistakenly logging in directly, they quickly backtracked and rerouted their attacks and messages through decoy computers abroad,” the paper reported, citing anonymous government officials.