Since it was released on Monday, Mr. Horowitz’s report has largely been interpreted through a political lens. Because it debunked Mr. Trump’s conspiracy theories and concluded that investigators had a legitimate and lawful basis to open the inquiry, some — like the F.B.I. director at the time, James B. Comey — have portrayed it as vindication.

But Senator Lindsey Graham of South Carolina, the Republican chairman of the committee, argued that the most important finding was instead the portrayal of a systemic and cultural failure of accountability at the F.B.I. that permitted grievous mistakes to make their way into filings seeking court permission to wiretap a former Trump campaign adviser, Carter Page.

Mr. Graham opened the hearing by acknowledging that the government of Russia — not Ukraine — sought to interfere with the 2016 election, and he did not quarrel with Mr. Horowitz’s finding that the F.B.I. had a legitimate basis to open a full counterintelligence investigation into links between Russia and people associated with the Trump campaign.

But he portrayed the wiretapping of Mr. Page as dubious and said it should have stopped after January 2017, when the F.B.I. had reason to lose confidence in evidence it used to obtain the initial court order targeting him. Mr. Horowitz’s findings about the wiretap applications should disturb all Americans, no matter their political leanings or the motivations behind the F.B.I. officials’ actions, he said.