Then what did the F.B.I. get wrong?

There were serious errors in the applications for secret eavesdropping warrants.

Mr. Horowitz found that F.B.I. officials appeared to discount evidence that did not support probable cause to wiretap Mr. Page while playing up information that seemed to justify one.

“That so many basic and fundamental errors were made by three separate, handpicked teams on one of the most sensitive F.B.I. investigations that was briefed to the highest levels within the F.B.I., and that F.B.I. officials expected would eventually be subjected to close scrutiny, raised significant questions regarding the F.B.I. chain of command’s management and supervision of the FISA process,” the report said.

The inspector general did not speculate whether the Foreign Intelligence Surveillance Court would have granted the application — renewed three times — to wiretap Mr. Page anyway. But he said the F.B.I. should review the actions of everyone who had a hand in drafting the applications. And he identified in each application where information verifying a fact was not included.

Relying on the F. B.I.’s information, the Justice Department first obtained court approval to wiretap Mr. Page in October 2016. The wiretap application portrayed Mr. Page, who had recently stepped down as a Trump campaign aide and had close ties to Russia, as a suspected unregistered agent of a foreign power. Mr. Horowitz found dozens of examples of missing or flawed documentation in the applications to wiretap Mr. Page.

The applications relied heavily on information provided by Christopher Steele, a British former intelligence agent whose research was funded by Democratic organizations. Mr. Steele told the F.B.I. that he based much of his information on a confidential source.

But when the F.B.I. interviewed that person, the source failed to back up some of Mr. Steele’s assertions, the report said. For instance, according to the F.B.I. interview, the source saw “nothing bad” about communications between the Trump team and the Kremlin, and never discussed WikiLeaks with Mr. Steele, according to the report. The F.B.I.’s failure to inform the court of those discrepancies was a serious error, Mr. Horowitz said.

“Despite the inconsistencies between Steele’s reporting and the information his primary sub-source provided to the F.B.I., the subsequent FISA renewal applications continued to rely on the Steele information, without any revisions or notice to the court,” the report stated.