FBI investigator Peter Strzok not only let Hillary Clinton off the hook, he may have used Democratic Party opposition research as an excuse to spy on Trump campaign advisers.

Strzok became such a political liability that special counsel Robert Mueller had to boot him off the Russia case, where he worked for nearly three months. Mueller made the move after the Justice Department’s inspector general pointed out text messages Strzok sent to a mistress, who also worked for Mueller, exhibiting a strong anti-Trump, pro-Clinton bias.

His misconduct has sent shock waves through Washington because in July 2016, just days after closing the Clinton email case he led, Strzok signed the document that opened the investigation into possible collusion between the Trump campaign and Russia. His fingerprints are all over both cases, one widely criticized as a whitewash and the other condemned by the president and many in his party as a witch hunt.

Potentially more disturbing is Strzok’s possible role in what many see as an even bigger scandal: the weaponizing of US intelligence against political opponents. Did he also sign documents asking a federal court to allow the FBI to spy on Trump advisers? It’s a critical question, because a so-called FISA document authorizing agents to monitor the communications of Trump adviser Carter Page, for one, reportedly was based at least in part on anti-Trump Russia propaganda promulgated in a dossier underwritten by the Clinton campaign — a partisan smear sheet that the FBI and Mueller have nonetheless used as a road map in their Russia probe.

In a Post interview, Page said he suspects Strzok, as the FBI’s No. 2 counterintelligence official, was also involved in applying for and obtaining the secret surveillance warrant on Page from the FISA court in September 2016. He adds that it’s “really interesting” that the dirty dossier found its way into Strzok’s orbit around the same time.

Indeed, according to an in-depth New York Times retrospective published earlier this year detailing the FBI’s two campaign investigations, Clinton subcontractor Christopher Steele briefed the FBI leadership about the findings in his now-discredited dossier in August 2016. Weeks later, the information landed “with Mr. Strzok and his team.”

“In late September, Mr. Steele heard back from his contact at the FBI,” the article continued. “The agency wanted to see the material he had collected ‘right away,’ ” while offering to pay him $50,000.

That month, a monitor was placed on Page.

Both the Senate and House are seeking the bureau’s FISA affidavits to determine to what extent they relied on the dirty Clinton dossier. But the FBI is stonewalling their requests. It’s also blocking FOIA requests by Page, who last month denied the dossier’s charges under oath, calling them politically motivated.

The FBI didn’t just target Page. It also targeted other Trump advisers. In fact, Strzok personally grilled Trump national security adviser Michael Flynn about his Russian ties at the White House just days after Flynn took office.

Last week, Mueller filed charges against Flynn for lying to Strzok about his contacts with the Russian ambassador during the presidential transition.

Strzok monitored intercepts of the Russian Embassy and already knew what Flynn and the Russian ambassador had discussed. So why did he need to ask him about what he already knew? Was he trying to trap him in a lie? Also, who leaked the intercepts to the press?

Strzok led the sham investigation of Clinton’s emails from start to finish, and helped draft her exoneration months in advance of her July 2, 2016, interview, which he personally supervised. He was the agent responsible for softening language then-FBI Director James Comey used in his July 5, 2016, statement clearing Clinton just ahead of the Democratic convention. He changed the rough draft of Comey’s announcement describing Clinton’s behavior as “grossly negligent” (a possible crime) to “extremely careless.” Strzok also was involved in the review of State Department emails discovered on Anthony Weiner’s laptop and gave Clinton her second pass just before the election.

The fix was in, and the fixer appears to have been a top G-man who, behind the scenes, sang the praises of the Democratic subject of his investigation and mocked the Republican subject of his other investigation.

Regardless of what you think about Trump, the Deep State — that is, the federal police and intelligence — shouldn’t get to pick and choose the leadership of this country. We are better than Thailand. If they are not held accountable, they will do it again.