Mr. Barr noted that it was a highly unusual step for the F.B.I. to investigate people associated with a presidential campaign. But he failed to capture the extraordinary nature of the 2016 election, which the inspector general report underscored, and omitted context relevant to F.B.I. officials’ decision to open the Russia inquiry.

For example, though Mr. Trump himself was not under investigation, his actions puzzled agents. Days after the stolen Democratic emails became public, he called on Russia to uncover more. Then news broke that his campaign had pressed to change the Republican Party platform’s stance on Ukraine in ways that were favorable to Russia.

Additionally, two Trump campaign advisers were already separately under federal investigation. And investigators stayed within protocol in opening the Russia inquiry, Mr. Horowitz found; the F.B.I.’s guidelines do not suggest extra steps or safeguards to take in opening such a sensitive and politically explosive investigation.

Mr. Barr said at the conference that another investigation he has opened, led by John H. Durham, the United States attorney in Connecticut, would resolve why the F.B.I. investigated four Trump campaign advisers.

Mr. Durham issued an unusual statement on Monday publicly disputing the inspector general’s findings about how the case was opened. Coming amid an active investigation with high-stakes political implications, the comment was reminiscent of the 2016 decision by James B. Comey, the F.B.I. director at the time, to publicly discuss the bureau’s investigation of Mrs. Clinton’s use of a private email server. He was eviscerated for publicly discussing an investigation that ended without charges.

The bureau’s applications for a court order approving the wiretap of a former Trump campaign adviser, Carter Page, and three renewals of it made up the bulk of Mr. Horowitz’s scrutiny of the Russia investigation but were a relatively small part of the sprawling inquiry.

Mr. Horowitz said the bureau had a reasonable basis to open the inquiry, but he uncovered mistakes made by the F.B.I. in working to wiretap Mr. Page, including exculpatory evidence that could have hurt the bureau’s ability to obtain its warrant, and said they were never adequately explained. Many of the problems centered on the F.B.I.’s reliance for the wiretap application on a notorious dossier of unverified, salacious information about Mr. Trump compiled by a British former spy, Christopher Steele.