For the last four years, the two watchdogs probing the FBI’s investigation of Donald Trump and his 2016 presidential campaign have disagreed on whether the bureau’s initial investigation was properly predicated.

The Inspector General of the Justice Department, Michael Horowitz, concluded that it was. The standard for opening a full FBI investigation in 2016 was a low bar, and the bureau met it after Australian officials passed on information about an encounter with a low level campaign aid named George Papadopoulos.  

When Mr. Horowitz released his devastating report in 2019, U.S. special counsel John Durham issued a brief statement disagreeing with the inspector general’s assessment, but did not provide further elaboration. Now he has.

“Our investigation gathered evidence that showed that a number of those closest to the investigation believed that the standard arguably had not been met,” Mr. Durham concludes.

That standard, laid out in the attorney general guidelines, says that a full investigation can be opened if there is an “articulable factual basis” for launching such a probe. Many believed there was not. 

The FBI’s legal attaché to the United Kingdom, which was involved in the probe, texted one of the supervisory agents at the time to complain that the basis for the investigation was “thin.”  

This same official in a 2019 interview with Mr. Durham’s team said that the senior bureau official, Peter Strzok, who opened the investigation known as Crossfire Hurricane said, “There’s nothing to this, but we have to run it to ground.”

Mr. Durham says his assessment was a red flag. He writes, “Strzok’s view would seem to dictate the opening of the matter as an assessment or, at most, as a preliminary investigation.”

These distinctions are crucial. An assessment or preliminary investigation would have imposed reasonable limits on the FBI’s investigation, such as a shorter timeline and not using more invasive investigation techniques. 

Mr. Durham also said the decision to launch the full investigation at the end of July 2016 violated other FBI guidelines.  

For cases that infringe upon legitimate First Amendment activities, such as presidential campaigns, FBI guidelines instruct, “the choice and use of investigative methods should be focused in a manner that minimizes potential infringement of those rights.”

Finally, the actual tip used to launch the probe was ambiguous and needed more scrutiny. Mr. Papadopoulos did not provide any specifics in his brief conversations with Australian diplomats. The referral to the FBI from those diplomats was deliberately vague.

Mr. Durham writes that shortly after opening the investigation, the bureau “learned from interviewing the Australian diplomats that there were reasons to be unsure about what to make of the information from Papadopoulos.”

Even more glaring, no one else from the intelligence agencies or the FBI had any evidence that Mr. Trump or his campaign had colluded with Russia when the bureau opened the full investigation.   

Mr. Durham adds that the FBI’s decision to open the Trump investigation on the thinnest ground stood in sharp contrast to its approach to investigations into the Hillary Clinton campaign in 2016. 

The FBI senior leaders placed so many limits on its investigation of the Clinton Foundation for the final months of the campaign that virtually nothing was probed in this period.

Another example involved a confidential human source for the FBI that made a small donation and nearly arranged a much larger one from a foreign power to the Clinton campaign. Instead of pursuing this lead, the bureau ended the investigation.  

All of these problems with the opening of Crossfire Hurricane are a scandal. What makes Mr. Durham’s report even more damning to the FBI is his meticulous documentation of how the bureau’s investigation consistently ignored or explained away leads and information that contradicted the Trump-collusion narrative.

This negligence included the bureau’s failed efforts to corroborate the infamous Steele Dossier, a set of private intelligence reports paid for by the Clinton campaign that alleged Mr. Trump and his campaign were engaged in an elaborate conspiracy with Russia.  

That dossier became the primary evidence the bureau submitted to a secret surveillance court to obtain a warrant to spy on Trump campaign adviser, Carter Page.

FBI agents interviewed the main sub-source for the dossier, a Russian national named Igor Danchenko. Mr. Danchenko was the target of an unresolved counter-intelligence investigation from 2010. It was dropped when the bureau incorrectly surmised that Mr. Danchenko left the country.

Mr. Durham’s review found “no indication” that the team investigating the Trump campaign ever tried to resolve the prior Danchenko espionage investigation before paying him as a confidential source. 

What’s more, the prior counterintelligence investigation into Mr. Danchenko was never shared with Justice Department attorneys responsible for applying for the surveillance warrant on Mr. Page.

“There was a complete failure on the part of the FBI to even examine — never mind resolve — the serious counterespionage issues surrounding Steele’s primary subsource, Igor Danchenko,” Mr. Durham concludes. 

As a result of that failure, the FBI never properly considered whether the Steele dossier itself was Russian disinformation.

Mr. Durham fails to give a definitive answer as to why the FBI’s leadership barreled ahead with an investigation that was going nowhere. He does, however, conclude that the bureau “discounted or willfully ignored material information that did not support the narrative of a collusive relationship between Trump and Russia.”

That narrative crippled the first two and a half years of Mr. Trump’s presidency and undermined the legitimacy of the 2016 election. Instead of informing the public that the speculation about Mr. Trump and Russia was without merit, the bureau’s leaders stoked a moral panic.