And that shows no signs of changing anytime soon, casting a long shadow over the fledgling government of President Donald Trump -- and what some say are its murky links with Russian President Vladimir Putin's Kremlin.
In July, US intelligence officials first reported that Moscow was interfering in the presidential election. They have since gone further, alleging Putin orchestrated a campaign to sway the vote in Trump's favor.
On Monday, White House national security advisor Michael Flynn was forced to resign over his private discussions with a Russian diplomat in December -- before Trump took office -- as then-president Barack Obama was preparing to impose sanctions on Moscow over the alleged election hacking.
After investigations by US spy agencies and the Justice Department, Congress is keen to get to the bottom of it all.
The issue has tainted Trump's election victory over Hillary Clinton, and raised fears that Moscow has found a new way to wreak havoc in its strategic rival by eroding confidence in elections, the cornerstone of American democracy.
The Republican property mogul-turned-president has made no secret of his desire to reset relations with Russia after years of tensions.
But Democrats in Congress smell blood in the water, and want deeper investigations into Russia's actions. And while some Republicans have said the issue should be laid to rest, others are starting to call for further inquiries.
The Senate Intelligence Committee has already opened a probe. And now they want to hear from Flynn.
"I would think that we should talk to General Flynn very soon and that should answer a lot of questions," said Roy Blunt, a Republican senator on the committee.
"What did he know? What did he do? And is there any reason to believe that anybody knew that and didn't take the kind of action they should have taken?"
Unanswered questions
After Trump's shock victory over Clinton in November, Obama began preparing the ground to retaliate against Russia over the hack claims.
But before he announced actions on December 30 -- more economic sanctions and the expulsion of 35 alleged Russian spies -- Flynn was reportedly telling Russia's US ambassador Sergey Kislyak not to worry.
Flynn reportedly conveyed the message that the Trump administration had a different view and would work to improve bilateral relations.
He resigned Monday after admitting he had not been completely honest with Vice President Mike Pence about his phone discussions with Kislyak.
But even with Flynn gone, the issue of Russia's interference remains a potent challenge for the Trump administration.
At least four Congressional committees are looking into various aspects of Russia's alleged interference, as well as Team Trump's links to Moscow.
Flynn is not the first member of Trump's inner circle to prompt speculation.
His former campaign director Paul Manafort was previously a consultant to Moscow-backed former Ukraine prime minister Viktor Yanukovych, and also worked with Russian oligarchs linked to Putin. He eventually stepped down.
Several Republicans have ridiculed the probes as "hysteria," but Democrats and some Republicans like top Senator John McCain have emphasized their importance.
Democratic Senator Mark Warner, also on the intelligence committee, said Tuesday that probing the issue "is more urgent than ever" after Flynn's resignation.
"These developments underscore how many questions still remain unanswered to the American people more than three months after Election Day, including who was aware of what, and when."
Ben Cardin, the top Democrat of the Senate Foreign Relations Committee, called Tuesday for an independent commission examining the hacking as well as what Trump himself knew.
There is also the unconfirmed dossier compiled by a former British intelligence agent with multiple explosive allegations, which transfixed political Washington for weeks.
Not only does it allege that Putin sought to give Trump's campaign a boost over Clinton's, it says people associated with Trump's campaign had ongoing communications with the Kremlin during the election.
Media reports in recent weeks say US intelligence officials have confirmed parts though not all of the dossier, including possibly communications between Trump allies and Russian officials.
'Incredibly tough'
Trump has repeatedly denied that Moscow helped him to his election victory, though he eventually conceded that they might have interfered in the campaign.
He also has made no secret of his admiration for Putin, tweeting in December that the Russian leader was "very smart."
On Tuesday, White House spokesman Sean Spicer insisted that Trump "has been incredibly tough on Russia."
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