Inquiry includes nature of any links between individuals associated with Trump campaign and Russian government.
The FBI is investigating Russia's interference in last year's U.S. presidential election, as well as any ties between Moscow and the Trump campaign, Director James Comey said on Monday.
"The FBI as part of our counterintelligence mission is investigating the Russian government's efforts to interfere in the 2016 presidential election," he told lawmakers while testifying on Capitol Hill.
"That includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government, and whether there was any coordination between the campaign and Russia's efforts," he said.
The public confirmation of an ongoing classified FBI investigation is incredibly rare, but Comey said it was in the public interest to do so.
This investigation will include an assessment of whether any crimes were committed, Comey said, noting that further details in the public setting would be sparse due the nature of the inquiry.
Depending on its results, the probe could be a boon to either party.
Democrats have long called foul over Russia's confirmed influence on the campaign last year, while Republicans have sought to downplay the effects.
U.S. intelligence officials concluded in January that Russia sought to influence the election in favor of President Donald Trump in a campaign directed by Russian President Vladimir Putin.
Trump has acknowledged the efforts, but has claimed they had "absolutely no effect" on the vote results.
The hacking of the Democratic National Committee (DNC) as well as Clinton campaign chairman John Podesta revealed unsavory details about how the supposedly neutral organization conspired to favor Clinton against populist Bernie Sanders to win the Democratic Party’s nomination for president.
Regular leaks of DNC and Clinton emails in the months leading up to the Nov. 8 poll cemented a popular narrative that the first female presidential nominee of a major party was not trustworthy.
Comey further said he has no information to substantiate Trump's allegations that former President Barack Obama had his "wires tapped" at Manhattan's Trump Tower.
"How low has President Obama gone to tapp my phones during the very sacred election process. This is Nixon/Watergate. Bad (or sick) guy!" Trump said in a series of March 4 tweets, misspelling "tap".
"We have no information that supports them," Comey said, referring to Trump's tweets.
"No individual in the United States can direct electronic surveillance of anyone," he added, noting that the process to do so would include approval from a judge.