US government agencies are rushing to tighten their computer security, after they have been hit by a hacking operation described as the worst in American history.
More than 40 organisations have been targeted, including the US Energy, Defense, State and Homeland Security departments.
Software giant Microsoft also confirmed it has been affected.
The hacking operation was uncovered last week, but investigators believe it has been going on for months.
US officials suspect it is carried out by Russian hackers, and warned of the 'grave' risk it poses to government and private networks.
What does it mean for America's national security?