The hackers were not easy to detect and just as difficult to erase them from the system. They had used the system for several months. Some people are not totally sure that they have been successfully removed. The FBI indicated that the break-in may have been the most sophisticated ever used against the U.S.
The computer system of the State Department, also non-classified, was broken into at about the same time. It appears that the hackers broke into the State Department's computer first, and from there were able to get into the White House's system. Officials in the State Department have been informed not to use that system for any classified information. Their computers had to be shut down completely for a while.
It is believed that the hackers were able to get back into the system after being restarted. This has led officials to realize that the attack is more serious than they thought.
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Source: http://abcnews.go.com/International/russian-hackers-white-house-system-months-officials/story
http://www.cnn.com/2015/04/07/politics/how-russians-hacked-the-wh/
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Graphics: Mine
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Keywords: Russian, hackers, White House, FBI, State Department, attack, computers