(Reuters) - The Republican chairman of the U.S. House Judiciary Committee on Friday subpoenaed a U.S. research firm founder to give a deposition on his hiring of a former British spy to compile a dossier on alleged links between U.S. President Donald Trump’s associates and Russia.

Representative Bob Goodlatte, a Republican from Virginia, announced the move on Twitter a day after receiving a letter in which lawyers for Glenn Simpson, the founder of Fusion GPS, said that their client “will not agree to an interview” requested earlier this week.

“As part of our joint investigation into decisions made by DOJ in 2016, today I subpoenaed Glenn Simpson to appear for a deposition,” Goodlatte posted on Twitter.

Goodlatte said in a separate Twitter post that former FBI Director James Comey, former U.S. Attorney General Loretta Lynch and other former Justice Department officials have been invited “as witnesses. Will subpoena them if necessary.”

Simpson’s lawyers also sent their letter to House Oversight Chairman Trey Gowdy, who is conducting with Goodlatte a probe into the Justice Department’s decision to investigate possible collusion between Trump’s 2016 presidential campaign and Russia.

The top Democrats on the committees, Elijah Cummings and Jerrold Nadler, in a statement accused the Republicans, who control Congress, of continuing “to abet the president as the walls close in around him.”

Fusion GPS was hired by lawyers for the Democratic Party and Hillary Clinton’s 2016 presidential campaign to conduct research on Trump, her opponent.

The firm in turn hired Christopher Steele, a former officer of MI6, the British foreign intelligence agency, to produce a dossier that outlined alleged Russian financial dealings with Trump, a longtime real estate developer, and included salacious personal details that remain unconfirmed.

The dossier was sent to the FBI, which already had launched an investigation into whether Russia interfered with the election and whether Trump campaign officials worked with Moscow to try to sway the outcome.

The Kremlin denies meddling in the election. Trump, who denies his campaign colluded with Russia, has called Steele’s work an “unverified and Fake Dirty Dossier” and has repeatedly denied any collusion with Russia.

In their letter, seen by Reuters, Simpson’s lawyers said Simpson already had been interviewed by the House Intelligence Committee, of which Gowdy is a member, and the Senate Judiciary Committee, which like all committees are led by Trump’s fellow Republicans.

“Part and parcel of this concerted effort by the president’s congressional allies has been a campaign of retaliation against the government’s whistleblowers, including our client Mr. Simpson, for their willingness to cooperate with U.S. law enforcement and for their exercise of their constitutional rights to free speech and political activity as American citizens,” the lawyers wrote.