The review uncovered a deeply dysfunctional and flawed process riddled with inaccuracies and material omissions. Investigators highlighted facts that made Mr. Page look suspicious while letting potentially exculpatory ones go unmentioned, and when they sought to renew the wiretap, they failed to correct earlier statements whose credibility had since come under serious question, the report found.

Justice Department lawyers who deal directly with the FISA court passed that misleading portrait onto the judges. While Mr. Horowitz’s findings placed most of the direct blame on a handful of case agents and their supervisors who worked directly with the raw evidence, his report also said senior officials bore responsibility for permitting systemic failures to fester.

Mr. Horowitz’s investigators found no evidence that political bias against Mr. Trump was behind the problems — as opposed to apolitical confirmation bias, gross incompetence or negligence. But the inspector general said the explanation the F.B.I. offered — that the agents had been busy with other aspects of the Russia investigation, and the Page FISA was a minor part of those responsibilities — was unsatisfactory.

Congress enacted FISA in 1978 to regulate the government’s use of domestic surveillance for national security investigations — those aimed at monitoring suspected spies and terrorists — as opposed to ordinary criminal cases. The law sets up a special court, made up of 11 sitting district court judges who are selected to serve staggered terms by the chief justice of the Supreme Court, and decide whether the evidence shows a target is probably a foreign agent.

Judge Collyer, who was appointed a federal district judge in Washington by President George W. Bush, has presided over the surveillance court since 2016. The chief justice of the Supreme Court, John G. Roberts Jr., had tapped her for a seat on it three years earlier, just before the disclosures by the former intelligence contractor Edward J. Snowden set off a wide debate over surveillance and privacy.

In 2018, government records show, the court fully denied only one of 1,080 final applications submitted under FISA to conduct electronic surveillance. However, the court also demanded unspecified modifications to 119 of those applications before approving them. There were 1,833 targets of FISA orders, including 232 Americans, that year.

National security wiretaps are more secretive than ordinary criminal ones. When criminal wiretap orders end, their targets are usually notified that their privacy has been invaded. But the targets of FISA orders are usually not told that their phone calls and emails have been monitored, or that their homes or businesses have been searched.