The co-founder of the firm that authored the infamous "Russian dossier" on President Trump will not testify before the Senate Intelligence Committee next week.

Fusion GPS co-founder Glenn Simpson and his attorneys said Simpson was traveling and wouldn't be available. But they also appeared to vent some frustration at the committee when they wrote that Simpson was "profoundly disturbed" that the hearings were becoming functions of "partisan agendas" in his opinion, Reuters reported.

The unsubstantiated dossier, sometimes referred to as the "Steele Dossier," has been a contentious item with a long, strange history in the ongoing investigations into Russian meddling in the 2016 elections.

Former British spy Christopher Steele was one of the primary authors of the document, as Fusion GPS was paid to put the dossier together as opposition research by political opponents of Trump in 2016.

The dossier was used, in part, as a basis to obtain an FBI warrant to surveil former Trump aide Carter Page, and intelligence officials briefed Trump on its contents just days before he was inaugurated in January.

Then, in March, Democrats on the House Intelligence Committee relied heavily on the dossier when questioning then-FBI Director James Comey and NSA Director Admiral Mike Rogers, as they explored a "spider web" of alleged connections between the Trump administration and Russian contacts in an open hearing.

Soon after, BuzzFeed published the full dossier. It wasn't used much as political ammunition as it became more clear many if not most of the claims in the dossier were not or would never be validated.

Page, having been surveiled partly because of the document's claims, has been a constant critic, calling it the "Dodgy Dossier," and has been very critical of the House Democrats who used it in their March questioning.

Meanwhile, a Russian banker who was named in the dossier has sued BuzzFeed for defamation, alleging the website published information it knew was false.