It's been three months since the U.S. State Department announced that hackers have embedded themselves in its computer network, but the agency still can't get rid of them. Help from the National Security Agency and outside contractors has been fruitless as the hackers, who were previously booted from the system, managed to work their way back in, according to the Wall Street Journal.

Sources who spoke to the newspaper reaffirmed what the State Department said when the hack was first made public in November: The hackers only have access to unclassified email networks. But the prolonged access could provide them with an unfiltered stream of valuable information on various government activities.

The duration of the intrusion is enough for investigators to suspect a rival nation state, in part because monitoring State Department communication requires a level of patience and know-how that’s beyond the typical hacker.

The attackers have had some form of access for at least seven months after the State Department announced in November that the infiltration began four months previously.

Investigators also cited a strain of malicious software that has been used by the Kremlin in the past, most notably when Moscow was assigned blame for a White House hack last year. The case proves that familiarity with malware doesn't make it easier to stop. The intruders slightly alter their method of entry each time they invade.

“We deal successfully with thousands of attacks every day,” State Department spokeswoman Marie Harf said in a statement to the Journal. “We take any possibility of cyberintrusion very seriously -- as we did with the one we discussed several months ago -- and we deal with them in conjunction with other relevant government agencies.”