Obama Emails Caught Up in Russian Hack

— -- The more authorities dig into the hacking of the State Department and the White House, it's becoming increasingly clear that the Russians were hunting for weaknesses.

Sources tell ABC News that some of President Obama's emails were caught up in the Russian cyberattack last October that led to a partial shutdown of some White House computers.

Hackers associated with the Russian government accessed computers that contained archives of unclassified White House communications -- including some involving the president, sources say. None of the president's devices were hacked, but some White House computers contained summaries of email exchanges the president may have had with people outside the administration.

While the attack did not involve top secret information, a number of officials say it shows an intense level of aggression by Russian operatives, who appear to be stepping-up hacks against the United States.

Defense Secretary Ash Carter has also described a new attack against the Pentagon earlier this year -- one remarkably similar in nature to the hacking of the State Department and the White House.

"Earlier this year, the sensors that guard DoD's unclassified networks detected Russian hackers accessing one of our networks. They'd discovered an old vulnerability in one of our legacy networks that hadn't been patched," Carter said Thursday during an address at Stanford University. "The cyber threat against U.S. interests is increasing in severity and sophistication."

Officials are concerned by the Russians' aggression -- and worried the Russians may be hoping that hacking non-classified computer systems could be a gateway into more sensitive systems and information.