(CNN)Overlooked in the controversy over Hillary Clinton's use of a private email server, is the fact that suspected Russian hackers have bedeviled State Department's email system for much of the past year and continue to pose problems for technicians trying to eradicate the intrusion.
Sources: State Dept. hack the 'worst ever'
Federal law enforcement, intelligence and congressional officials briefed on the investigation say the hack of the State email system is the "worst ever" cyberattack intrusion against a federal agency. The attackers who breached State are also believed to be behind hacks on the White House's email system, and against several other federal agencies, the officials say.
The issue is relevant because one criticism of the Clinton private email use is that it was likely less secure than emailing within the State Department's system. But the hack shows that State's system has major security issues. The State system, investigators believe, was compromised in the past year, likely after Clinton left the State Department.
Last November, the State Department shut down its email system over a weekend to try to improve security and block the intruders.
At the time, the agency tried to send a re-assuring message that "activity of concern" by possible hackers only affected its unclassified email system.
But officials say that even a breach of the unclassified system poses major security risks, because sensitive information of value to foreign intelligence agencies is routinely shared in non-classified emails.
Russian hackers, likely working for the Russian government, are suspected in the State Department hack.
The FBI has been investigating the hacking activity.
In part because of the Russian attack on State, U.S. intelligence officials have increased their warnings about Russian hacking activity in the U.S.
James Clapper, director of National Intelligence, told a Senate hearing last month the "Russian cyber threat is more severe than we have previously assessed."
Clapper's comments were in part prompted by the intrusion at State and other federal agencies, according to one official briefed on the matter.
Former Secretary of State Hillary Clinton said Tuesday that her use of a private email server didn't include any classified information. She said the computer server hosting her emails was in a private building protected by the U.S. Secret Service.
Officials say that doesn't necessarily protect against hackers who don't need physical access to the server. In a statement released by Clinton's office after the press conference Tuesday, a spokesman said there was "no evidence" the server was ever hacked.