The United States is suspecting Russian actors in the recent Pentagon unclassified network breach affecting 4000 people working for the Joint Staff (emphasis added):

The Defense Department disclosed the attack shortly after it occurred, but only in recent days have investigators traced it to Russia. Officials said the complexity and advanced nature of the hack strongly suggested that a foreign government was responsible.

“This attack was fairly sophisticated and has the indications . . . of having come from a state actor such as Russia,” said a U.S. official who spoke on the condition of anonymity to discuss details of the investigation.

The cyberattack on the Joint Staff, which coordinates operations among the branches of the armed forces, is similar to one last fall that successfully penetrated unclassified e-mail systems at the White House and the State Department. In that case, U.S. officials said the trail also led to hackers thought to be working for the Russian government.

Even so, officials cautioned that it is difficult to pinpoint the origin or perpetrator of such hacks. “Attribution in this business is near impossible. Rarely are you ever able to say with 100 percent certainty” who was behind a particular incident, the official said.

It is obvious the attack was perpetrated by a nation state, of which there are only a limited number of countries capable and willing to pull off this type of operation.