The Justice Department’s inquiry into the origins of Russiagate has now expanded into a criminal matter, raising alarm bells among intelligence officials, Democratic leaders, and media pundits who promoted the theory of a Trump-Russia conspiracy.

There is no doubt that Donald Trump would like to exact political revenge on those behind the Russia probe, and it is fair to be skeptical of his Department of Justice. But it would be a mistake to reflexively dismiss the inquiry, which is led by US Attorney John Durham and overseen by Attorney General William Barr. The public deserves an accounting of what occurred. And given the intrusion of the nation’s intelligence’s services into domestic politics, a failure to learn lessons and enact safeguards could leave future candidates, especially on the left, vulnerable to similar investigations.

For more than two years, the FBI investigated a presidential campaign and then sitting president as a conspirator or agent of Russia. The story engulfed US media and political energy and had major consequences on domestic US politics and foreign relations. The probe found not only no Trump-Russia conspiracy, but barely even any contact between the two sides suspected of conspiring. Carrying a Russian passport (as the Russians in the Trump Tower meeting did), or falsely suggesting in an e-mail that you are acting at the Kremlin’s behest (as the British music publicist who arranged that meeting did), does not mean that you are actually working with the Russian government. Mueller, ultimately, showed no evidence that they—or any other suspected Kremlin intermediary—were Kremlin intermediaries. This helps explain why, as the report found, Kremlin officials trying to reach out to the Trump campaign after its election victory “appeared not to have preexisting contacts and struggled to connect with senior officials around the President-Elect.”

And even with this all-consuming investigation now over, we still do not have a firm understanding of how it began. We are told that the FBI launched the probe after receiving a tip that a Trump campaign volunteer, George Papadopoulos, may have been given advance notice by a mysterious professor, Joseph Mifsud, that Russia had damaging information on Hillary Clinton. Former FBI director James Comey has declared that Mifsud is “a Russian agent.” But Mueller, conspicuously, never applied that label to Mifsud. Even more conspicuously, Mueller failed to indict him on any charges, despite claiming that Mifsud lied to FBI agents when they interviewed him in February 2017. Compounding the mystery, Barr and Durham reportedly obtained Mifsud’s cell phones during a recent trip to Italy, where he went missing in November 2017. In court filings, attorneys for Michael Flynn, Trump’s former national security adviser, are now contending that Mifsud is a Western intelligence asset involved in a scheme to frame Flynn and the Trump campaign by association.

As the journalist T.A. Frank notes in Vanity Fair, there are strong reasons to doubt the theory that Mifsud was involved in an effort to set up the Trump campaign. But the presence of one potentially false conspiracy theory does not negate the fact that Mifsud helped trigger the certifiably false conspiracy theory that the Trump campaign conspired with the Russian government. It would be most welcome to find out exactly how that occurred.

It is also important to find out the extent to which the FBI relied on the Steele dossier. We have yet to receive a credible explanation for why intelligence officials thought it was appropriate to take cues from an unverified collection of lurid conspiracy theories about Trump—all paid for by his political opponent. What has already been revealed is damning enough. The FBI cited the Steele dossier to obtain a surveillance warrant on former Trump campaign aide Carter Page in October 2016, telling the court that it “believes that [Russia’s] efforts are being coordinated with Page and perhaps other individuals associated with,” the Trump campaign. Its source for that wild supposition was Steele, whom it described as “Source #1” & “credible.”

Then there is the role of the CIA under John Brennan. Multiple news reports make clear that the CIA is a principal focus of Barr and Durham’s inquiry. In breaking the story of the expanded criminal inquiry, The New York Times includes the curious claim that Durham has asked interview subjects “whether C.I.A. officials might have somehow tricked the F.B.I. into opening the Russia investigation.”

Although there are limitations on how much we can make of one sentence, that is a tantalizing clue pointing to Brennan. The former CIA director has taken credit for launching the Russia investigation, telling Congress in May 2017 that his own “concerns” about “contacts between Russian officials and US persons” associated with the Trump campaign “served as the basis for the FBI investigation to determine whether such collusion-cooperation occurred.” The Barr-Durham inquiry will hopefully uncover what exactly Brennan’s “concerns” were.

Brennan’s role should be scrutinized because he not only said he helped initiate the Trump-Russia probe but because he managed the US intelligence response to alleged Russian interference. It was Brennan who personally delivered to the White House a top-secret envelope containing the claims of a Kremlin mole that Vladimir Putin had personally ordered an interference operation to install Trump. And amazingly, we have recently learned not only the mole’s identity but the curious circumstances of his exit from Russia. The mole, a mid-level Kremlin official named Oleg Smolenkov, initially refused CIA efforts to remove him, prompting concerns in Langley about his trustworthiness, and that he was possibly a double agent. Reporting from The New York Times makes clear that he left Russia only after leaks about a Kremlin mole began appearing in US media.

After his outing, journalists found Smolenkov living in the Virginia suburbs under his own name. All of this makes for a curious profile for someone who had supposedly revealed what The Washington Post dubbed “the crime of the century.”

In addition to helping trigger the Russia probe and overseeing the intelligence response, Brennan oversaw the hasty production of the Intelligence Community Assessment (ICA) that purportedly validated it. By claiming that Putin ordered an influence campaign to elect him, the January 2017 ICA helped cast a criminal shadow over Trump’s presidency just days before he took office. A series of unsubstantiated leaks from anonymous officials—about Flynn, about the Steele dossier, and about fictitious or overblown Trump-Russia contacts—continued that pattern as the Mueller investigation dragged on. Perhaps the most extraordinary example came in February 2017, when The New York Times reported that the US investigators had obtained “phone records and intercepted calls” showing that members of Trump’s campaign and other associates “had repeated contacts with senior Russian intelligence officials in the year before the election.” Four months later, Comey testified that this was “not true.”

If the FBI had investigated President Barack Obama for more than two years on the false allegation of conspiring with or being an agent of a foreign power, Democratic leaders would rightfully demand a full inquiry. It would set a dangerous precedent for liberals to now reject an effort to get answers only because those answers would not be politically expedient. If left unchecked now, the same intelligence services that involved themselves in domestic politics in 2016 could do so again against progressive candidates on similarly spurious grounds.

The unfortunate reality is that under Trump, Democratic leaders and intelligence officials used the Russia investigation as a political weapon against his presidency. Now that it has proven baseless, Trump and his supporters have legitimate grounds to uncover how it began. The fact that Trump will use Russiagate’s failure as a political weapon is exactly why us skeptics on the left warned that its evidentiary holes would help him. Rather than complaining, those who brought us Russiagate should accept responsibility for handing Trump that opportunity, and work to ensure that the national security state does not receive opportunities to intrude again.