This is stated in a joint statement by the Office of the Director of National Intelligence, the Federal Bureau of Investigation (FBI), the National Security Agency and the US Infrastructure and Cyber Security Agency.
“The source of the targeted sustained threat is likely to come from Russia and is responsible for the vast majority or all of the recently detected cyber attacks against both government and non-government networks,” the text says.
In December, US intelligence agencies issued a joint statement that it had identified a "significant cyber incident" that "affected networks within the federal government."
Later, US President Donald Trump announced that China could be behind a major cyber attack on the United States.