A retired top Pentagon official said Thursday that Russian hackers seized complete control of an unclassified email system in a devastating 2015 cyberattack thought to be revenge for U.S. sanctions. Former Chairman of the Joint Chiefs of Staff Martin Dempsey told CBS News that the cyberattack caused chaos in the Pentagon, with hackers taking over an entire email system in less than an hour. Hundreds of senior officers had their passwords stolen and the network had to be taken down, Dempsey said. Since the email system was unclassified, officials now believe Russia was not seeking to get valuable intelligence but to wreak havoc at the Pentagon in retaliation for U.S. sanctions over Russia’s occupation of Crimea. According to Dempsey, it took the Pentagon about two weeks to rebound from the attack and replace the system’s hardware and software.