Russian hackers carried out a cyber attack against the unclassified email system of the Pentagon's Joint Chiefs of Staff in late July, a report says.
On Thursday, NBC News cited unnamed sources as saying that no classified information had been taken or revealed during the "sophisticated cyber intrusion" that affected about 4,000 military and civilian personnel around July 25.
The Pentagon confirmed that the Joint Chiefs' email system was taken offline following the attack pending an investigation but declined to elaborate.
"We continue to identify and mitigate cyber security risks across our networks," Pentagon spokeswoman Lieutenant Colonel Valerie Henderson said, adding, "With those goals in mind, we have taken the Joint Staff network down and continue to investigate. Our top priority is to restore services as quickly as possible.”
It was not clear, however, if the spear-phishing attack was sanctioned by the Russian government.
"As a matter of policy and for operational security reasons, we do not comment on the details of cyber incidents or attacks against our networks," she elaborated.
Spear-phishing attacks usually trick people into opening infected emails that steal their network credentials and spread through a network.
The hackers used what the NBC sources called an automated system that rapidly gathered massive amounts of data and within a minute distributed all the information to thousands of accounts on the Internet.
Also last month, US officials confirmed that some 21 million US government employees, contractors and others had personal information compromised in two breaches. The officials, however, have declined to identify the source of the cyber attacks.