LOS ANGELES -- Russia, enraged by the Obama administration's sanctions over Ukraine, could strike back at the U.S. via cyber warfare -- then deny the attack ever took place, a former top intelligence official said Tuesday. Russia, considered the second country most adept at cyber warfare after the U.S., could use an attack on American computer systems as a way to seek revenge on the U.S., said Richard Clarke, a top counter-terrorism and intelligence official for both the Clinton and George W. Bush administrations. Former Defense Secretary Leon Panetta, who joined Clarke and former National Security Agency Deputy Director Chris Inglis at the Milken Institute Global Conference here, says Russia is no different than the U.S. when it come to its attitude toward cyber warfare. It's a key element of its strategy. The Russians already used it during in dealing with Georgia and Crimea, Panetta said. If Russia decides on military action against Ukraine, "cyber would be part of that attack element," Panetta says. Hurricane Sandy showed how a cyber attack could effect the U.S. when it tore through the mid-Atlantic in 2012. It knocked out the power grid, paralyzing much of the region. Motorists couldn't pump gasoline. Hospitals were unable to function normally. Clarke warned that private corporations face the biggest threat from cyber warfare. He says companies are under frequent attack these days by often backed by governments like Iran or China. He says in once case, had spent $1.2 billion developing a project, only to lose it all to hackers on a single afternoon. The first step for companies is openness. Often, he says companies aren't even aware that they had been hit. "You have to realize you've been hacked and develop a a strategy different than today," he says. "You're not going to keep them out. Don't kid yourself." Milken 2014: Russia could wage cyber war on U.S.