Russian hackers are attacking the Pentagon’s Joint Staff unclassified email system, leaving thousands of Department of Defense (DOD) workers without email for nearly two weeks, a DOD spokeswoman confirmed.

Officials believe Moscow may have orchestrated the “sophisticated cyberattack,” which infiltrated the Joint Chiefs of Staff email system sometime around July 25, according to multiple reports.

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The Pentagon’s cybersecurity team quickly shut down the system and has been working since to revamp and relaunch the network. Roughly 4,000 workers have been without email during that time.

Officials said no classified networks were accessed.

"Joint Staff unclassified networks for all users are currently down," a DOD spokeswoman said. "We continue to identify and mitigate cybersecurity risks across our networks. With those goals in mind, we have taken the Joint Staff network down and continue to investigate."

The digital intrusion is possibly tied to a Russian hacking group, known as APT29, that was recently profiled in a report from security firm FireEye, NBC News reported.

That team uses a tactic called Hammertoss that allows hackers to clandestinely communicate with malware that has already infected a computer system, allowing it to remain undetected. The strategy, FireEye said, reveals a “discipline and consistency” that is nearly unmatched by other top-notch hacking groups.

An official told CNN that the methods used to crack the Pentagon’s network were something government investigators had not seen previously.

The Joint Staff cyberattack is far from the first time Russia-based hackers were suspected in major intrusions at top government agencies.

Defense Secretary Ashton Carter revealed in April that Russian hackers had broken into the DOD’s unclassified networks for a brief moment. Late last year, suspected Moscow-backed hackers also got into both the State Department and White House networks, accessing sensitive materials such as President Obama’s personal schedule. Both agencies spent months trying to kick out the cyber invaders.

DOD officials hope to have the Joint Staff email system running again on Thursday.

"Our top priority is to restore services as quickly as possible," the DOD spokeswoman said.

Lawmakers said the incident is further evidence of the government's deficient cyber defenses.

Sen. Steve Daines (R-Mont.) called the reports "deeply concerning."

"It’s unacceptable that the Obama administration has not yet addressed these serious holes in our nation’s security," he said in a statement Thursday.

He is urging Congress to move on cybersecurity legislation. The upper chamber on Wednesday was forced to punt a major cyber bill after failing to reach a bipartisan deal in the final days before the August recess.

"We need to take concrete steps to prioritize the strengthening of our nation’s cybersecurity infrastructure," Daines said.

— Updated 4:42 p.m.