Officials are trying to determine whether a cyberattack was behind outages at websites affiliated with the North Atlantic Treaty Organization and whether hackers were attempting to counter a Warsaw summit that is addressing both cyberspace dangers and Russian aggression. Officials said there is no concrete evidence the websites were knocked out by hacking. But some officials suspect that Russian-backed hackers could be responsible for the website outage that took down a pair of websites affiliated with NATO’s Allied Transformation Command based in Norfolk, Va. The Russian Defense Ministry was not available for comment. Other Russian officials did not immediately respond to a request for comment.