The hackers that recently infiltrated the Joint Staff’s unclassified email network completely changed tactics just one week after the Pentagon thwarted a prior attempt to crack the system, said National Security Agency (NSA) Director Adm. Michael Rogers.

“We totally forestalled it,” Rogers told an audience during remarks at the Woodrow Wilson International Center for Scholars on Tuesday, The Wall Street Journal reported. “Within a week I watched them totally change the structure that they used.”

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The novel attack caused the military to take down the email server of 4,200 officials for two weeks in late July and early August.

Rogers described a sophisticated hack, with a “different scheme and maneuver that I had not seen before.”

It’s believed Russian hackers were behind the digital assault. Some have even tied the campaign to a Moscow-based hacking group known as APT29. The team uses a previously unseen tactic known as Hammertoss, in which hackers clandestinely communicate with embedded malware through dummy Twitter accounts.

Sen. Ted Cruz (R-Texas), who is running for the GOP presidential nomination, has even alleged the attack was timed to coincide with secretive meetings between Qasem Soleimani, a major general in the Iranian army, and Russian leaders in Moscow.

The Russian cyber moves come as Congress prepares to vote on a deal to put limits on Iran's nuclear weapons program in exchange for lifting international sanctions. Some believe scaling back the sanctions will enable Iran and Russia to strengthen ties.

Rogers would not comment on who the Pentagon believes was behind the incident.

But the NSA head did describe a cyber campaign targeting the Pentagon that was widespread, ongoing and sophisticated.

“The campaign went against dozens of networks, segments of the network within the network,” Rogers said.

The Joint Staff hack was the one time the group was “able to achieve that level of penetration,” he added.