The Pentagon confirmed that the Joint Chiefs' email system was taken offline pending an investigation but declined to elaborate.
  
"We continue to identify and mitigate cybersecurity risks across our networks," Pentagon spokeswoman Lieutenant Colonel Valerie Henderson said.
  
"With those goals in mind, we have taken the Joint Staff network down and continue to investigate. Our top priority is to restore services as quickly as possible.     

"As a matter of policy and for operational security reasons, we do not comment on the details of cyber incidents or attacks against our networks."    

NBC, citing unnamed sources, said it was not clear if the attack had been sanctioned by the Russian government.