Glenn Simpson, co-founder of the opposition research firm that compiled a salacious anti-Trump dossier, on Tuesday invoked his Fifth Amendment right not to testify before a House committee investigating the Justice Department‘s actions during the 2016 presidential election.

Mr. Simpson’s turn on Capitol Hill prompted President Trump to launch a Twitter screed against the journalist-turned-boss of the political intelligence firm Fusion GPS.

“Conflict between Glen Simpson’s testimony to another House Panel about his contact with Justice Department official Bruce Ohr. Ohr was used by Simpson and Steele as a Back Channel to get (FAKE) Dossier to FBI. Simpson pleading Fifth.” Catherine Herridge. Where is Jeff Sessions? — Donald J. Trump (@realDonaldTrump) October 16, 2018

The president took another swipe at Mr. Ohr, whose wife Nellie is a Fusion GPS employee.

Is it really possible that Bruce Ohr, whose wife Nellie was paid by Simpson and GPS Fusion for work done on the Fake Dossier, and who was used as a Pawn in this whole SCAM (WITCH HUNT), is still working for the Department of Justice????? Can this really be so????? — Donald J. Trump (@realDonaldTrump) October 16, 2018

Mr. Simpson’s decision not to speak was not surprising. Last week, his legal team legal team issued a statement presaging his invocation of the Fifth Amendment, accusing the committee of running a partisan probe.

Still, House Judiciary Committee Chairman Bob Goodlatte called Mr. Simpson’s refusal to answer questions “disappointing,” noting that he has already appeared before lawmakers on two occasions.

“Today, we expected Mr. Simpson to answer questions that have not been explored by other congressional committees to help advance our joint investigation,” Mr. Goodlatte, Virginia Republican, said. “Instead, Mr. Simpson has refused to cooperate with our investigative team and has denied the American people answers to important questions.”

Mr. Simpson is one of several key players in the dossier drama scheduled to testify on Capitol Hill this week. James Baker, the FBI’s former top lawyer, is scheduled to appear on Thursday and Mrs. Ohr is set for Friday.

The dossier, authored by former British spy Christopher Steele, was funded by the Democratic National Committee and Hillary Clinton’s presidential campaign. It is full of salacious, unverified allegations about Mr. Trump’s ties to Russia. Special counsel Robert Mueller’s probe into ties between the Trump campaign and Russia.

The so-called Steele dossier was used to obtain a Foreign Intelligence Surveillance Act application to spy on Trump campaign adviser Carter Page.

Republican lawmakers have pointed to the fact Mrs. Nellie Ohr worked for Fusion GPS during the 2016 election as evidence that the Justice Department was politically motivated to sink the Tump campaign.