DAVID RIVKIN AND ELIZABETH PRICE FOLEY: Mueller’s Fruit of the Poisonous Tree: It makes no difference how honorable he is. His investigation is tainted by the bias that attended its origin in 2016.

Special counsel Robert Mueller’s investigation may face a serious legal obstacle: It is tainted by antecedent political bias. The June 14 report from Michael Horowitz, the Justice Department’s inspector general, unearthed a pattern of anti-Trump bias by high-ranking officials at the Federal Bureau of Investigation. Some of their communications, the report says, were “not only indicative of a biased state of mind but imply a willingness to take action to impact a presidential candidate’s electoral prospects.” Although Mr. Horowitz could not definitively ascertain whether this bias “directly affected” specific FBI actions in the Hillary Clinton email investigation, it nonetheless affects the legality of the Trump-Russia collusion inquiry, code-named Crossfire Hurricane.

Crossfire was launched only months before the 2016 election. Its FBI progenitors—the same ones who had investigated Mrs. Clinton—deployed at least one informant to probe Trump campaign advisers, obtained Foreign Intelligence Surveillance Court wiretap warrants, issued national security letters to gather records, and unmasked the identities of campaign officials who were surveilled. They also repeatedly leaked investigative information.

Mr. Horowitz is separately scrutinizing Crossfire and isn’t expected to finish for months. But the current report reveals that FBI officials displayed not merely an appearance of bias against Donald Trump, but animus bordering on hatred. Peter Strzok, who led both the Clinton and Trump investigations, confidently assuaged a colleague’s fear that Mr. Trump would become president: “No he won’t. We’ll stop it.” An unnamed FBI lawyer assigned to Crossfire told a colleague he was “devastated” and “numb” after Mr. Trump won, while declaring to another FBI attorney: “Viva le resistance.”

The report highlights the FBI’s failure to act promptly upon discovering that Anthony Weiner’s laptop contained thousands of Mrs. Clinton’s emails. Investigators justified the delay by citing the “higher priority” of Crossfire. But Mr. Horowitz writes: “We did not have confidence that Strzok’s decision to prioritize the Russia investigation over following up on [the] investigative lead discovered on the Weiner laptop was free from bias.”

Similarly, although Mr. Horowitz found no evidence that then-FBI Director James Comey was trying to influence the election, Mr. Comey did make decisions based on political considerations. He told the inspector general that his election-eve decision to reopen the Clinton email investigation was motivated by a desire to protect her assumed presidency’s legitimacy.

The inspector general wrote that Mr. Strzok’s text messages “created the appearance that investigative decisions were impacted by bias or improper considerations.” The report adds, importantly, that “most of the text messages raising such questions pertained to the Russia investigation.” Given how biases ineluctably shape behavior, these facts create a strong inference that by squelching the Clinton investigation and building a narrative of Trump-Russia collusion, a group of government officials sought to bolster Mrs. Clinton’s electoral chances and, if the unthinkable happened, obtain an insurance policy to cripple the Trump administration with accusations of illegitimacy.