The FBI learned North Korea was responsible for last year's cyber-attack on Sony Pictures within a month after the case was first reported, a senior agency official said Tuesday, rejecting allegations that it could be an act by insiders.
  

"We were absolutely positive in a very short period of time" that the North Korean government was behind the attack, Joseph Demarest, assistant director of the FBI's cyber crime division, was quoted as saying at a briefing, according to news reports.
  

Demarest was also quoted as saying the Sony Pictures hack would not be a "one off."
  

North Korea has denied its involvement in the Sony hack.
  

Cyber-security experts have also questioned the FBI's conclusion that the North was responsible, suggesting the possibility that disgruntled Sony insiders or somebody else launched the attack and made it look like it was done by North Korea.
  

The FBI has rejected such speculation.
  

Meanwhile, the FBI, the State Department and the Justice Department announced a reward of up to $3 million for information leading to the arrest or conviction of Evgeniy Mikhailovich Bogachev, a Russian national accused of involvement in a major cyber-racketeering enterprise.
  

Bogachev, thought to be at large in Russia, has been under indictment in connection with his alleged role as administrator of a hacking program believed to be responsible for the theft of more than $100 million from businesses and consumers in the U.S. and around the world, the agency said. (Yonhap)