Three months after it confirmed that its non-classified email system was breached, the US State Department is still struggling to root out hackers from its network, according to a Wall Street Journal report.

As per the report, despite repeated scanning of the network, investigators (which include government officials, outside contractors, as well as the NSA) still see signs of the hackers on the department's computer systems. Each time a malicious software is blocked, the hackers tweak it slightly to attempt to sneak past defenses.

As it stands, the extent to which the network has been made hacker-free isn't yet clear, although portions of the unclassified email system, such as remote email access, are still occasionally taken down.

Another thing that is concerning investigators is that hackers are trying to re-infect the computer systems that have been cleaned, although it isn't yet known whether, and to which extent, they have been successful at that.

Unlike corporate breaches, where networks are usually taken offline for cleanup on weekends, the State Department's breach is proving difficult to cleanup given its large size and key function -- making sure the US knows what's happening around the world.

There has been no official word on who is behind the breach, and how much data the hackers have taken.