The CIA had top-level intelligence last August that Russian President Vladimir Putin personally ordered an operation to help Donald Trump win the US presidential race, the Washington Post reported Friday.
The intelligence shocked the White House and put US security chiefs on a top-secret crisis footing to figure out how to react.
But amid confidence that Democrat Hillary Clinton still had the election in the bag and worries over president Barack Obama himself being seen as manipulating the election, the administration delivered warnings to Moscow but left countermeasures until after the vote, the Post reported.
After Trump's shock victory, there were strong regrets among administration officials that they had shied from tough action.
"From national security people there was a sense of immediate introspection, of, 'Wow, did we mishandle this,'" a former administration official told the newspaper.
The Post said that as soon as the intelligence on Putin came in, the White House viewed it as a deep national security threat. A secret intelligence task force was created to firm up the information and come up with possible responses.
They couldn't do anything about embarrassing WikiLeaks revelations from hacked Clinton emails. The focus turned to whether Moscow could disrupt the November 8 vote itself by hacking voter registration lists or voting machines, undermining confidence in the vote tally itself.
Worried about making the situation worse, the administration put off retaliating, and instead delivered stiff warnings directly to the Russians not to go farther.
At least four direct warnings — Obama to Putin, spy chief to spy chief, and via top diplomatic channels — appeared to have an impact, officials told the Post. They believe that Moscow pulled back on any possible plans to sabotage US voting operations.
"We made the judgment that we had ample time after the election, regardless of outcome, for punitive measures," a senior administration official told the Post.
Options to retaliate were on the table early: more crippling sanctions on the Russian economy, leaking information that would embarrass Putin diplomatically, and launching cyberattacks on Russian infrastructure were high on the list.
But Trump's shock victory dampened the response.
Obama took modest measures at the end of December, expelling 35 Russians and adding to existing sanctions. He also, according to the Post, authorized a plan to place cyberattack implants in the systems of critical Russian infrastructure. But it remains unclear, the Post said, whether Trump has followed through with that.