Although the U.S. State Department's unclassified email system experienced a data breach three months ago, the government has not been able to ouster the hackers from its network, according to the Wall Street Journal.

From activity found on the network by outside contractors, it appears that the attackers are staying a step ahead of the government, modifying the tools they're using to avoid security measures, the Journal cited three sources close to the investigation as saying.

While the State Department hasn't laid blame for the attack on any group in particular, sources told the newspaper the attack has hallmarks that signal the Russian government's handiwork.  

The malware used has, in the past, been tied to Russia. Although the government doesn't know the extent of the data taken, sources told the Journal that stolen information included emails related to the Ukraine crisis.