The co-founder of a Washington opposition research firm that produced a dossier of salacious claims involving Donald Trump has spent hours speaking to Senate investigators in a closed-door meeting.
Glenn Simpson's lawyer, Josh Levy, emerged from the day-long private appearance and said his client had "told Congress the truth and cleared the record on many matters of interest".
Mr Levy said Mr Simpson appeared voluntarily and had so far been the only witness to be interviewed by the Senate Judiciary Committee as it looks into Russian interference in the 2016 US presidential election.
The sheer length of Mr Simpson's appearance - far longer than President Trump's son-in-law spent earlier this summer with Senate and House of Representatives intelligence committees - reflected the intrigue on Capitol Hill surrounding the dossier and the origins of the document.
Mr Simpson's firm, Fusion GPS, hired a British intelligence officer who produced a dossier containing allegations of ties between Mr Trump and his associates and Russia.
Mr Simpson kept the identities of the firm's clients confidential during his appearance before Congress, Mr Levy said.
The document attracted public attention in January when it was revealed that FBI director James Comey had briefed Mr Trump about its existence soon before he was inaugurated as president.
It is unclear to what extent the allegations in the dossier have been corroborated or verified by the FBI, since the bureau has not discussed it publicly.
"Fusion GPS is proud of the work it has conducted and stands by it," Mr Levy said in a statement.
He said the "investigation into Mr Simpson began as a desperate attempt by the Trump campaign and its allies to smear Fusion GPS because of its reported connection to the Trump dossier".
Leaders of the Judiciary Committee said last month that they were negotiating private appearances for Donald Trump Jr,, who has attracted scrutiny for accepting a June 2016 meeting with Russians at which he expected to receive damaging information about Hillary Clinton, and for Paul Manafort, the former Trump campaign chairman.
No dates have been announced for their appearances.
"Following up on comments from certain Senate Judiciary Committee members who have noted Mr Simpson's co-operation with this investigation, I would like to add that he is the first and only witness to participate in an interview with the committee as it probes Russian interference in the 2016 election," Mr Levy said.