That might be a reasonable response if we were confident the Page investigation represented an outlier or aberration. The chilling reality, however, is that we have no idea whether that’s the case.

At a Senate Judiciary Committee hearing last week, Senator Marsha Blackburn, Republican of Tennessee, zeroed in on this point. When she asked Mr. Horowitz whether finding mistakes in a FISA application was “a fairly unusual occurrence,” he responded, “I would hope so.”

Americans deserve a stronger assurance than “hope” that their Fourth Amendment rights are being respected. The sheer quantity of serious defects in the FISA applications targeting Mr. Page — which officials consistently told Mr. Horowitz received far more review than normal, because agents understood the applications would doubtless attract controversy and scrutiny — raises an obvious and disturbing question: If they’re this sloppy with a target involved in a presidential campaign, how bad is it in ordinary cases, which the public will never learn about and which are unlikely to ever be the topic of congressional hearings?

We needn’t worry so much about that, of course, if the defects of the Page warrants were products of political animus against the Trump campaign. But the report provides very little reason to think that’s the case. The case for supposing bias is the culprit here leans heavily on the former F.B.I. agent Peter Strzok, now notorious for a voluminous history of text messages denigrating Mr. Trump and suggesting that he would not become president because “we will stop it.” But while Mr. Strzok played a supervisory role in the earliest stage of the Page investigation, it’s hard to tie him to the specific problems Mr. Horowitz identifies. As the report notes, Mr. Strzok “was not the primary or sole decision maker on any investigative step” and at one point opposed FISA monitoring of another Trump campaign staff member that case agents proposed. Moreover, the problems Mr. Horowitz documented in the initial FISA application filed under Mr. Strzok’s watch were significantly less serious than the outrageous omissions and misrepresentations to the court that occurred in the subsequent applications to renew the wiretap, after Mr. Strzok’s role in the investigation had ended.

With one significant exception — an F.B.I. lawyer responsible for improperly altering an email related to the final renewal application — Mr. Horowitz didn’t find signs of Mr. Strzok’s intense animus among others who worked on the FISA warrants. The report notes that among the huge quantity of internal communications reviewed, the inspector general identified “a small number of text messages and instant messages” in which members of the investigation team “discussed political issues and candidates,” but that these “did not raise significant questions of potential bias or improper motivation.”

If there’s an explanation for the errors Mr. Horowitz documents suggested by his reports, it’s not political bias. It’s confirmation bias.

The F.B.I.’s interest in Mr. Page — and its suspicions that he might be a Russian intelligence asset — predated his involvement in presidential politics. He had reportedly been the target of a FISA warrant in 2014 and was the focus of yet another counterintelligence investigation opened in April 2016 by the F.B.I.’s notoriously Trump-friendly New York field office, months before the bureau started an inquiry into potential links between the Trump campaign and Russia’s election interference operation. When investigators got wind of Christopher Steele’s notorious dossier, which made Mr. Page a pivotal figure in a “well-developed conspiracy of cooperation” between Mr. Trump and the Kremlin, it would have seemed like confirmation of what they already suspected.

Having adopted this theory, investigators began to exhibit classic signs of confirmation bias, readily absorbing new information that fit the model they’d built, while overlooking or explaining away facts that didn’t fit. The worst misrepresentations to the court that Mr. Horowitz uncovered are sins of omission — new information the bureau obtained as the investigation progressed that should have led it to question previous representations it had made to the court.

The many layers of review FISA applications go through — laid out in a set of rules known as the Woods Procedures — were ill equipped to detect this sort of problem, because the Woods Procedures focus on confirming that facts in the application match documents in the F.B.I.’s case file. But you can’t fact check a claim that doesn’t exist — which means the process is bad at detecting important information that has been left out. Officials who reviewed later applications also told Mr. Horowitz that they typically focused on the new information in each submission. That means assertions they’d made early on ended up effectively being taken for granted: Nobody was revisiting early assumptions to see whether they still held up in the face of new data.

If this explains why the Page investigation went increasingly off the rails, it’s an explanation that has little to do with partisan politics at its heart. But that would mean there’s little reason to think the Page investigation is special in this respect. There’s an urgent need, then, for the inspector general to do more such “deep dives” and figure out just how pervasive the problem really is.

Fortunately, the inspector general is already taking a first step in this direction, having begun a review that will “examine the F.B.I.’s compliance with the Woods Procedures in FISA applications that target U.S. persons.” But in itself, that’s not enough: While Mr. Horowitz found violations of the Woods Procedures in the Page case, they weren’t the most serious distortions. Those occurred precisely because the Woods Procedures aren’t well calibrated to catch material facts that get left out. To do that, you’d need to do the kind of intensive and comprehensive case-by-case review conducted in the Horowitz review, not just run Woods vetting a second time to see whether the results tally.

Doing this sort of deep dive for a representative sample of FISA applications will, of course, be both expensive and extremely time consuming. But it’s well worth it to find out just how badly our surveillance state is broken.

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