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.

Grassley asked Fusion GPS to identify its client and reveal details of its arrangement with Steele, who heads the London-based firm, Orbis Business Intelligence. The Republican also pressed Fusion on its interactions with the FBI, which was said to have made an informal agreement in October to pay Steele for future research on Trump.

“When political opposition research becomes the basis for law enforcement or intelligence efforts, it raises substantial questions about the independence of law enforcement and intelligence from politics,” Grassley wrote in the letter, dated March 24. (RELATED: Senator Wants Answers From Opposition Research Firm Behind Trump Dossier)

[dcquiz]

The committee has also asked FBI Director Jim Comey about its discussions with Fusion GPS and Steele.

In their response letter to Grassley, Fusion GPS’s lawyers, Joshua A. Levy and Robert F. Muse, argue that the firm does not have to provide the information because the work is protected under the First Amendment, by attorney-client privilege, and by confidentiality agreements.

“The March 24 letter calls for information and documents protected by the First Amendment rights, attorney-client privilege, attorney work product, and contractual rights (e.g., confidentiality agreements) of Fusion GPS and/or its clients,” the lawyers write. “Thus, so as to preserve those privileges and rights, we will not otherwise be responding to the questions enumerated within the March 24 letter.”

“Much of Fusion GPS’ work is covered by the attorney-client privilege and/or the attorney work product doctrine, which Fusion GPS lacks the authority to waive,” they added.

It is unclear on what basis Fusion GPS is invoking attorney-client privilege.

Follow Chuck on Twitter