The FBI, Secret Service and U.S. intelligence agencies are all involved in investigating the breach, which they consider among the most sophisticated attacks ever launched against U.S. government systems. The intrusion was routed through computers around the world, as hackers often do to hide their tracks, but investigators found tell-tale codes and other markers that they believe point to hackers working for the Russian government. A spokesman for the National Security Council declined to comment. Neither the U.S. State Department or the Russian immediately embassy responded to a request for comment.


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