McAfee didn't find any concrete evidence linking the attacks to North Korea, but Alperovitch said the company is convinced the attack was conducted by the government of the reclusive Asian nation or a group closely allied with it.
The same conclusion was reached by a South Korean government investigation into the attacks.
North and South Korea remain technically at war, having never signed a peace agreement at the close of the Korean War in 1953. The border between the two neighbors is one of the most heavily fortified in the world.
The 2011 attacks showed an additional degree of sophistication over the 2009 attacks, said McAfee. The March incident involved 14 target websites, less than a third of the 40 sites hit in 2009, and this time included no websites in the U.S.
"This time they dropped all the U.S. targets," he said. "They know taking down WhiteHouse.gov serves no purpose, and taking down NYSE.com doesn't impact the U.S. economy because they're hitting the website, not the trading system."
The narrower range of targets and enhancements to the command and control systems indicate the attackers learned several lessons from their first attempt in 2009, said Alperovitch. The lessons learned this time could further be developed to ensure greater success the next time around.