Steve Bannon, former chief strategist to US President Donald Trump, has been summoned to testify before a grand jury, US media report.
He was reportedly subpoenaed by former FBI director Robert Mueller, who is leading an inquiry into alleged collusion with Russia during the 2016 election campaign.
Mr Bannon appeared separately on Tuesday before a Congressional panel.
Congress is holding its own inquiry into the allegations.
Mr Bannon was subpoenaed last week, the New York Times reports, quoting an unnamed person with direct knowledge of the matter.
However the summons could be a negotiating tactic by Mr Mueller to persuade Mr Bannon to agree to be questioned by investigators in the less formal setting of the special counsel's offices in Washington, the paper's source added.
He was appointed by the US justice department in May of last year to oversee an investigation into possible Russian meddling in the 2016 presidential election.
US intelligence agencies, including the CIA and NSA, concluded with "high confidence" in 2016 that Russia was behind an effort to thumb the scale of the US election for Mr Trump, reports BBC.