A federal grand jury on Friday approved the first charges in the investigation into alleged Russian meddling in the 2016 U.S. presidential election, a source briefed on the matter told Reuters.
The indictment was sealed under orders from a federal judge so it was not clear what the charges were or who the target was, the source said, adding that it could be unsealed as early as Monday.
The filing of charges by the grand jury in Washington was first reported on Friday by CNN, which said the target could be taken into custody as soon as Monday.
U.S. intelligence agencies concluded in January that Russia interfered in the election to try to help President Donald Trump defeat Democratic candidate Hillary Clinton through a campaign of hacking and releasing embarrassing emails, and disseminating propaganda via social media to discredit her campaign.
Special counsel Robert Mueller, a former director of the Federal Bureau of Investigation, is investigating whether Trump campaign officials colluded with those Russian efforts.
"If the Special Counsel finds it necessary and appropriate, the Special Counsel is authorized to prosecute federal crimes arising from the investigation of these matters," Deputy Attorney General Rod Rosenstein said in a May 17 letter appointing Mueller.
Sources familiar with Mueller’s investigation said he has used that broad authority to investigate links between Trump aides and foreign governments as well as possible money laundering, tax evasion and other financial crimes.
Peter Carr, a spokesman for Mueller, declined to comment on Friday.
Trump, a Republican who was elected president last November, has denied allegations that his campaign colluded with Russians and condemned investigations into the matter as "a witch hunt".
The Kremlin has denied the allegations.
Mueller's investigation also includes an effort to determine whether Trump or any of his aides tried to obstruct justice.
The special counsel's team has conducted interviews with former White House chief of staff Reince Priebus, former spokesman Sean Spicer and other current and former White House officials.
In July, FBI agents raided the home in Virginia of Trump's former campaign manager, Paul Manafort, whose financial and real estate dealings and prior work for a pro-Russian political party in Ukraine are being investigated by Mueller's team.
Mueller was appointed to lead the investigation a week after Trump fired FBI Director James Comey, who was heading a federal probe into possible collusion with Russia.
Trump initially said he fired Comey because his leadership of the FBI was inadequate and hurt morale, but in a later interview with NBC he cited "this Russia thing" as his reason.
The Russia investigation has cast a shadow over Trump's nine-month-old presidency and widened the partisan rift between Republicans and Democrats.
Republican lawmakers earlier this week launched investigations to examine several of Trump's longstanding political grievances, including the FBI probe of Hillary Clinton's emails and her alleged role in a sale of U.S. uranium to a Russian firm.
Mueller's team has also investigated Michael Flynn, who was an adviser to Trump's campaign and later briefly served as his national security adviser.
Flynn was fired from that post in February after misleading Vice President Mike Pence about the extent of his conversations with Russian ambassador Sergei Kislyak last year.
While he was on Trump's campaign team, Flynn also had a $600,000 contract from a Turkish businessman to help discredit U.S.-based Turkish cleric Fethullah Gulen, accused by Turkey's government of instigating a failed coup in July 2016.
Former CIA director James Woolsey, who was also an adviser to the Trump campaign, has alleged that Flynn discussed with the businessman and two Turkish government ministers the idea of covertly spiriting Gulen out of the United States to face charges in Turkey.
Jonathan Franks, a spokesman for Woolsey, said on Friday that Woolsey and his wife have been in communication with the FBI and Mueller's team about the claim.
Woolsey and his wife, Nancye Miller, "have responded to every request, whether from the FBI, or, more recently, the Office of the Special Counsel," Franks said in a statement.
Flynn has previously denied through a spokesperson that such a plan was ever discussed.
Reuters reported on Thursday that Woolsey and his wife last year pitched a $10 million project to the same Turkish businessman who had agreed a smaller contract with Flynn. They did not win a contract.
Bidding for a lobbying or consulting contract with a foreign company or government is not illegal but Flynn came under scrutiny because he waited until March to retroactively register with the Justice Department as a foreign agent for the work he did on the Gulen project.