US intelligence agencies believe Russia acted covertly to boost Donald Trump in the election race, US officials have told leading newspapers.
A report in the New York Times says the agencies had "high confidence" about Russian involvement in hacking.
A CIA assessment reported by the Washington Post made similar findings.
But Mr Trump's team dismissed the CIA line, saying: "These are the same people that said Saddam Hussein had weapons of mass destruction."
Russian officials have repeatedly denied the hacking accusations.
On Friday, US President Barack Obama ordered an investigation into a series of cyber-attacks, blamed on Russia, during the US election season.
The hacks targeted emails at the Democratic Party and a key aide to presidential nominee Hillary Clinton.
In October, US government officials pointed the finger at Russia, accusing it of meddling in the campaign.
Now, senior administration officials quoted by the New York Times say they are confident that Russian hackers also infiltrated the Republican National Committee's computer systems as well as those of the Democratic Party, but did not release information gleaned from the Republican networks.
Intelligence agencies say the Russians passed on the Democrats' documents to WikiLeaks, the Times reported.
Democrats reacted furiously when email accounts of the Democratic National Committee and Mrs Clinton's campaign chairman, John Podesta, were hacked.
The Podesta emails were revealed by WikiLeaks and posted online.