The Justice Department watchdog is investigating whether the alarming errors revealed in a wiretap application for one of President Trump’s former campaign aides were one-offs or indicative of systemic flaws in the government’s use of its most intrusive, secretive and powerful surveillance tool.

The probe grew from Inspector General Michael Horowitz’s lengthy review of the FBI’s investigation into Trump’s 2016 campaign, months of work that included the most extensive look ever at a national security surveillance application. His findings, outlined in a newly released report, highlight imperfections in the national security surveillance regime and raise questions about whether a fundamental overhaul is in order.

“Time and again during our . . . interviews,” Horowitz told lawmakers after the release of his report, “FBI managers, supervisors and senior officials displayed a lack of understanding or awareness of important information concerning many of the problems that we identified.’’

Although Horowitz did not find evidence that political bias influenced the FBI’s decision in the fall of 2016 to seek a wiretap on the former campaign aide, Carter Page, he identified 17 inaccuracies and omissions in the initial application and three renewal requests, a stunning number for a sensitive case that FBI officials knew would be subject to close scrutiny. He has recommended the entire FBI chain of command be assessed for their performance failures. The revelation has drawn outrage from Republican lawmakers and sparked a robust debate among legal and policy experts.

The power to monitor a suspect on American soil in national security cases dates to 1978, when Congress passed the Foreign Intelligence Surveillance Act, or FISA, to rein in abuses from the domestic spying scandals of the era.

“If this represents a decent percentage of typical errors, then FISA cannot continue without radical reform,” Jack Goldsmith, who headed the Justice Department’s Office of Legal Counsel in the George W. Bush administration, told the Lawfare podcast following the release of Horowitz’s report.

Other former senior Justice Department officials have tried to put the shortcomings in perspective. The monitoring of Page was part of a broader investigation opened by the FBI in July 2016 to ascertain whether Russia was trying to influence that fall’s presidential election through current or former members of the Trump campaign. It was a chilling prospect, and former FBI leaders told Horowitz that the bureau had to see whether there was a serious threat to national security.

“In general, if you look at any complex, fast-moving, high-stakes, high-stress process under a microscope after the fact, you will find errors,” said David Kris, an assistant attorney general for national security during the Obama administration and an associate deputy attorney general in the Bush administration, speaking on the same podcast. “Turn over all the rocks, and you’re going to find some worms. . . . But these worms found here, I think, are worse than you would normally find.”

He said Horowitz’s broader review of FISA applications in espionage and terrorism probes will be “a good litmus test” of whether the Page case is “an outlier” or whether the process is broken.

In response to Horowitz’s findings, FBI Director Christopher A. Wray has announced measures to tighten the rules for FISA surveillance requests. Among other things, he’s requiring that all information bearing on the reliability of a confidential informant be included in the FBI’s wiretap application and verified by the informant’s handler.

Such steps don’t go far enough, say critics who have long contended that the FISA process lacks important safeguards to prevent abuse.

“If the inspector general identified such serious concerns about an investigation that everyone knew was so sensitive, just think what happens in less politically charged cases, especially those involving Muslims, other racial minorities and activists,” said Hina Shamsi, director of the American Civil Liberties Union’s National Security Project. “FBI oversight of its own activities is not enough to guard against the secrecy and one-sided approval process that characterizes the system and breeds abuse.”

Congress, she said, needs to transform the FISA process to make it more transparent and to ensure “meaningful opportunity” to challenge the government’s allegations in applications and in cases in which wiretap evidence is used in a criminal prosecution.

Senate Judiciary Committee Chairman Lindsey O. Graham (R-S.C.) indicated that Congress may have to act, saying during a hearing to dissect Horowitz’s findings that lawmakers “have a task at hand here to make sure this never happens again — to hold people accountable, change our laws, save the FISA court if we can.”

To obtain a FISA order, the government must persuade a judge it has probable cause to believe the target is an agent of a foreign power. The proceedings are classified, and no one stands before the judge to argue on behalf of the target. So over the years, procedures have been created to ensure that the information in the applications, prepared with the help of Justice Department attorneys, is accurate.

But Horowitz found that the FBI agents who prepared the application and renewals made significant errors. For one thing, the application relied in large part on allegations about Russian contacts with Page. Those allegations appeared in a dossier compiled by a former British spy, Christopher Steele, whose research was financed by the Democratic Party.

Steele had provided tips to the FBI in the past. FBI officials said Steele’s reporting had been “corroborated and used in criminal proceedings,” which overstated the significance of his work. That statement was not approved by Steele’s FBI handler, either.

One of the most damning findings in the report was that an FBI lawyer had retroactively altered an email to make it look as if Page was not a source for the CIA, when in fact the agency had told the FBI as early as August 2016 that it had a previous relationship with him. That information could have softened suspicions against him and, a Justice Department attorney told Horowitz, “would have been a significant fact to disclose.”

And in a January 2017 interview, Steele’s primary source gave the FBI information that contradicted assertions made in the initial application, which was based on Steele’s dossier. But the agents failed to inform the Justice Department attorneys, who might have advised against renewing the surveillance. Rather than report the inconsistency, the FBI said the primary source was “truthful and cooperative.”

James Baker, who was the FBI’s general counsel at the time, said Horowitz’s findings “indicate errors and omissions that were utterly unacceptable. I can’t defend them, and I won’t defend them.”

Baker, who left the bureau in May 2018 and is one of the most seasoned FISA practitioners to have served in government, has seen warts in the process before — and flagged them to the FISA court.

In 2000, when Baker was the lead Justice Department supervisor handling FISA applications, the government reported errors in more than 75 requests related to terrorism investigations. The following year, Baker alerted the court to errors in another set of applications.

Those disclosures prompted a set of rules — informally called the “Woods procedures” after Michael J. Woods, the FBI lawyer who wrote them — which today guide the FBI’s FISA submissions. Baker helped Woods draft the guidelines.

The most significant measure taken back then, Baker said, was a judge’s barring of an agent who had signed several of the applications from appearing before the FISA court again. It devastated the agent’s career but also “changed how FBI agents thought about the FISA process,’’ he said.

Baker said the inspector general’s report may serve the same deterrent purpose “because no agent will ever want this type of scrutiny to fall on them.”

Horowitz has not mapped out a timeline for his new review, but lawmakers have indicated they are eager to assess his findings and act on them as appropriate.

“And I hope,” Graham said, “this chapter in American history is never repeated.”