James Clapper, the director of national intelligence, is appearing before the Senate intelligence committee, where lawmakers' questions will expose the underlying debate over the future of US-Russian relations.
Clapper also addressed the Russian interference when appearing before the Senate Armed Services Committee last week ahead of the report's release.
The declassified report explicitly tied Russian President Vladimir Putin to the hacking of email accounts of the Democratic National Committee and individual Democrats like Hillary Clinton's campaign chairman, John Podesta.
The report lacked details about how the US learned what it says it knows, such as any intercepted conversations or electronic messages from Russian leaders, including Putin. It also said nothing about specific hacker techniques or digital tools the US may have traced back to Russia in its investigations.
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Putin spokesman Dmitry Peskov told reporters yesterday that the Kremlin still believes the US accusations of election hacking have no substance.
"We categorically rule out the possibility that Russian officials or official bodies could have been involved. We are tired of such accusations. This is beginning to remind us of a full-fledged witch hunt."
According to US intelligence agencies, Russia provided the emails to WikiLeaks. The website's founder, Julian Assange, denies that is the case, but Democratic and Republican members of Congress have largely backed the accusation and many have demanded a sterner response.
Yesterday, Assange called the report on hacking a politically motivated "press release" and said it provided no evidence that Russian actors gave WikiLeaks hacked material.
The GRU is Russia's military intelligence agency. The FSB is the main successor to the Soviet-era KGB.
Yesterday, the US levied economic sanctions against five Russians in connection to a 2012 US law punishing Russian human rights violators.
Americans are now banned from doing business with the men and any assets they may have in the United States are now frozen.