President Trump called on the Department of Justice and FBI to reveal who paid opposition research firm Fusion GPS to produce the infamous anti-Trump dossier of research compiled last year by former British spy Christopher Steele.

“Officials behind the now discredited ‘Dossier’ plead the Fifth,” Trump wrote on Twitter on Saturday. “Justice Department and/or FBI should immediately release who paid for it.”

Trump was referring to Fusion co-founders’ Peter Fritsch and Thomas Catan pleading the Fifth during interviews with the House Intelligence Committee earlier this week.

The committee has recently ramped up its efforts to investigate the dossier. In addition to the subpoena for Fusion’s co-founders, the panel earlier this month subpoenaed Fusion’s bank, TD Bank, for records that would disclose the identities of its clients. (RELATED: Fusion GPS Fights House Subpoena For Bank Records, Which Would Reveal Its Dossier Client)

Fusion was working for a Hillary Clinton ally when it began work on the dossier. There have been few clues about who the client might be, and Fusion has resisted all attempts to find out who it is.

It is unclear whether the FBI and Justice Department know who paid Fusion GPS to produce the dossier, which BuzzFeed published on Jan. 10. But the agencies have reportedly used the document as part of the investigation into possible collusion between the Trump campaign and Russian government.

Steele, a former MI6 agent, reportedly met with FBI agents last July to provide briefings on his investigation of Trump. And the bureau reportedly struck an informal agreement with Steele last October to pay him to continue his investigation. It is unclear if the retired spy was paid by the FBI for his sleuthing, though there have been reports that he was reimbursed by the bureau for some expenses.

Republican lawmakers have raised questions about the FBI’s involvement with Steele and the dossier, mostly because it is the product of a politically-motivated opposition research campaign. Republicans on the House Intelligence and Senate Judiciary Committees have pressed the FBI and DOJ about whether those agencies vetted the dossier prior to using it as part of its investigation into the Trump campaign.

So far, the agencies have not provided responses to those questions.

For its part, Fusion has largely refused to cooperate with congressional committees conducting Russia-related investigations.

Glenn Simpson, one of the co-founders of Fusion and the point-man on the dossier project, refused to identify Fusion’s clients in August, when he met with the Senate Judiciary Committee for 10 hours. The firm says that revealing its clients would hurt its business while undercutting the constitutional rights of its clients to engage in political activities.

On Friday, Fusion’s attorneys filed a request in federal court in Washington asking a judge to place a temporary restraining order to prevent the release of the records. Lawyers for the House committee filed a rebuttal on Saturday. The judge in the case is expected to respond with a decision next week.

Follow Chuck on Twitter