The opposition research firm that hired a former British spy to dig up dirt on Donald Trump last year is refusing to answer questions recently posed to it by the Senate Judiciary Committee.

The firm, Fusion GPS, is protected by attorney-client privilege and the First Amendment, its attorneys said in a letter to the committee.

The chairman of the Senate panel, Iowa Sen. Chuck Grassley, sent a letter to Fusion GPS last month inquiring about the Trump research project.

Fusion GPS, which was founded by former Wall Street Journal reporter Glenn Simpson, hired ex-MI6 agent Christopher Steele in June to uncover any Trump ties to Russia. Steele produced a series of memos laying out various allegations about Trumpworld’s ties to Russian government officials. For its part, Fusion GPS was on the payroll of an unidentified Democratic ally of Hillary Clinton’s.