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.
In 2018, government records show, the court only fully denied 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.
And when people are prosecuted for crimes based on evidence derived from ordinary criminal wiretaps, the defendants and their lawyers are usually allowed to see what the government told judges about them to win approval for that surveillance, giving them the opportunity to argue that investigators made mistakes and the evidence should be suppressed.
But defense lawyers, even those with security clearances, are not shown FISA applications for their clients. As a result, there is no prospect of second-guessing in an adversarial court setting to keep F.B.I. agents scrupulous about how they portray the evidence when seeking to persuade FISA judges to sign off on putting a target under surveillance.
In the absence of that disciplining factor, the Justice Department and F.B.I. have developed internal procedures that are supposed to make sure that the evidence presented in FISA applications is accurate and includes any facts that might undercut the government’s case. But that system failed in the Page wiretaps, Mr. Horowitz’s report showed.