Attorney: Glenn Simpson did not reveal clients for Trump 'dossier' to investigators He spent nearly 10 hours behind closed doors answering questions on Tuesday.

 -- The private investigator behind the now-infamous Trump “dossier” spent nearly 10 hours behind closed doors answering questions from Senate investigators on Tuesday, but his attorney says he did not reveal who paid his company for the research.

Glenn Simpson, a former Wall Street Journal reporter who cofounded the private research firm Fusion GPS that was hired initially by Republicans and later worked with Democrats to explore then-candidate Donald Trump’s past, was called to Capitol Hill to answer questions about the 35-page document that featured uncorroborated and salacious allegations about the business mogul’s ties to Russia.

"Mr. Simpson told Congress the truth and cleared the record on many matters of interest to congressional investigators,” Simpson’s attorney Josh Levy told reporters after the marathon session concluded.

He said the firm remains “proud” of the work and “stands by it.”

As special counsel Robert Mueller’s high-profile probe of those alleged ties heats up, Sen. Chuck Grassley (R-Iowa), the chairman of the Senate Judiciary Committee, has led an effort to uncover the funders of and sources for that controversial document and its — so far — largely unverified claims.

Grassley made it clear that he intended to question Simpson’s motivations and challenge the findings laid out in the document, which was largely prepared by the former MI6 agent Christopher Steele.

“We will also pursue details about Mr. Simpson’s role in this event and the creation and circulation of the dossier that started this whole controversy,” Grassley said in a hearing in July.

While Simpson’s attorney said his client provided significant details about his firm’s findings, he did not reveal the identities of those who paid for his research.

Simpson “kept the identities of Fusion GPS’ clients confidential,” Levy said in his statement. “Fusion GPS represents businesses, individuals and, occasionally, political clients on both the right and the left. When those clients want Fusion GPS to keep their identities confidential, Fusion GPS honors that commitment without exception - just as law firms and businesses do all over the country.”

Grassley’s staff took pains to keep the location of Tuesday’s session a secret, and the committee did not release a statement when it concluded. Levy left it to the committee to determine whether to publicize the details Simpson shared.

“The Committee has a transcript of the interview,” Levy said. “We are not permitted to have a copy. The Committee has the right to disclose the transcript, if it wishes to do so.”

The so-called “dossier” has remained a source of intrigue to those questioning Trump’s alleged ties to Russia, and a source of anger for the president. When BuzzFeed obtained and published the document in January, little more than a week before of Trump’s inauguration, he was outraged.

“It’s all fake news. It’s phony stuff. It didn’t happen,” Trump said at a press conference. “And it was gotten by opponents of ours, as you know, because you reported it and so did many of the other people. It was a group of opponents that got together — sick people — and they put that crap together.”

According to Levy, however, the multiple ongoing investigations into the issues raised in the “dossier” is proof that the firm’s work has made a significant impact.

“This 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, which Watergate prosecutor Richard Ben-Veniste and others have said is a ‘roadmap for an investigation,’” Levy said. “Today, a special counsel and three congressional committees are investigating ties between the Russian government and the Trump campaign. Fusion GPS is proud of the work it has conducted and stands by it.”