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.