Two top officials with Fusion GPS, the company responsible for the creation of the so-called "Trump dossier," appeared before the House Intelligence Committee on Wednesday, but invoked their Fifth Amendment rights and didn't answer questions, according to a lawyer who represents them.

Peter Fritsch and Thomas Catan, two partners in Fusion GPS, presented themselves to the committee to satisfy a subpoena, but went no further, as was expected.

The dossier appears to be of increasing importance as congressional investigators looking into Russia's meddling in the 2016 election have cleared away scores of interviews, reviewed thousands upon thousands of pages of documents, yet are unable to get to the bottom of who paid for the document while other issues surrounding the dossier don't seem to have been answered to the satisfaction of investigators.

The dossier was having an impact late in the 2016 campaign and even the transition period for the Trump administration, and was published in full by Buzzfeed in January. Most of the claims in the dossier, which appears to show President Trump's ties to Russia, remain unconfirmed.

The developments come just two weeks after Sen. Richard Burr, chairman of the Senate Intelligence Committee, expressed his frustration about not being able to speak with or interview former British spy Christopher Steele, who is believed to have authored most of the dossier and therefore is called by his name.

In August, another founder of Fusion GPS, Glenn Simpson, interviewed with the Senate Intelligence Committee for 10 hours. Simpson is known for his time as an investigative reporter with the Wall Street Journal.

Fusion's lawyer, Josh Levy, called the use of a subpoena an "indignity."

"This disparate treatment is an abuse of power and unethical," Levy continued in a statement. "Nor is it a necessary tool for Congress to use in order to obtain information. Even the Senate Watergate Committee spared witnesses this indignity."

In particular, Levy directed his ire at House Intelligence Committee Chairman Devin Nunes, who has stepped aside from leading the committee's Russia investigation, but remains as chair of the panel.

"Any attempt to change either the narrative or a congressional committee's focus will not change the facts, which we hope all serious investigators will learn," Levy added.

Besides both the House and Senate Intelligence Committees looking into the dossier, Senate Judiciary Chairman Chuck Grassley is also digging into the provenance of the document.

Grassley has voiced concerns that as Simpson was assembling the document in 2016, he might have shared some of his material with the British intelligence services, who then could have passed on that information to American intelligence agencies without specifying their source.

If the FBI had a copy of the document, Grassley's theory goes, any notification from British intelligence services about the information would have looked like independent corroboration of allegations, when instead, Simpson would have been the sole source.

Some conservative media outlets have even theorized that the dossier may have been the original factor that sparked investigations by the House and Senate intelligence committees.

In an open hearing in March, Democratic members of the House Intelligence Committee referenced the dossier numerous times as a basis for questioning then-FBI Director James Comey and Director of the National Security Agency Mike Rogers.