According to an intelligence report, Russian President Vladimir Putin sought to help Donald Trump win the presidential election.
The unclassified report says Vladimir Putin “ordered” a campaign aimed at influencing the election.
Moscow has not commented, but Russia has previously denied the claims.
After being briefed on the findings, Donald Trump stopped short of accusing Russia of interfering, saying only that the election outcome was not affected.
The president-elect later went on to blame “gross negligence” by the DNC for “allowing” the hacking to take place.
Following the report’s findings, the US Department of Homeland Security announced that voting machines and other election databases would be classified as “critical infrastructure” and given more protection from cyber-attack.
The 25-page report says that Russia developed a “clear preference” for Donald Trump.
Russia’s goals, the document added, were to “undermine public faith” in the US democratic process and “denigrate” Hillary Clinton, harming her electability and potential presidency.
The report said: “We assess Russian President Vladimir Putin ordered an influence campaign in 2016 aimed at the US presidential election.”
The unclassified version contained no detailed evidence of Vladimir Putin’s alleged role, but it said Russia’s actions included: hacking into the email accounts of the DNC and top Democrats; using intermediaries such as WikiLeaks, DCLeaks.com and Guccifer 2.0 persona to release the information acquired from the hackings; using state-funded propaganda and paying social media users or “trolls” to make nasty comments.
The identities of the Russian agents allegedly directly responsible for the hack are known to US authorities but have not been released publicly, reports say, citing intelligence sources.
The report, a cut-down version of the classified document presented to President Barack Obama on January 5, was released shortly after intelligence chiefs briefed Donald Trump on their findings.
Since winning the election on November 8, Donald Trump has repeatedly questioned US intelligence claims of Russian hacking.
In a statement after the briefing, Donald Trump declined to single out Russia, but said he had “tremendous respect for the work and service done” by those in the US intelligence community.
He said: “While Russia, China, other countries, outside groups and people are consistently trying to break through the cyber infrastructure of our governmental institutions, businesses and organizations including the Democrat National Committee, there was absolutely no effect on the outcome of the election.
“Whether it is our government, organizations, associations or businesses we need to aggressively combat and stop cyber-attacks. I will appoint a team to give me a plan within 90 days of taking office.”
The meeting at Trump Tower involved National Intelligence Director Gen. James Clapper, CIA Director John Brennan and FBI Director James Comey.
Donald Trump, who will be inaugurated on January 20, described their talks as “constructive”.