A “state-of-the-art cyber attack” by alleged Russian hackers driven the US department of defence to shut down its combined staff unclassified email system, leaving a large number of workers without emails for almost two weeks, the US media reported.

US officials who spoke on Thursday on conditions of anonymity changed about 4,000 and said that the intrusion occurred around military and civilian employees, Xinhua reported, citing NBC News.

The officials said the hackers organized the cyber attack on Pentagon through social networking reports, including they couldn’t determine whether the Russian government was behind the assault.

In a different hacking event this past year, the White House’s computer system was broken by alleged Russian hackers and apparently read the US President Barack Obama’s unclassified emails.