President Biden on Sunday announced an FBI investigation into the Secret Service’s astounding failure to protect Donald Trump at the rally in Butler, Pa., promising the probe would be “thorough and swift.”

But it’s folly to trust a federal law enforcement agency to obliterate interagency niceties by exposing all the foibles and flubs of fellow G-men.

Instead, we’ll likely see a vintage DC investigation along these lines: Please tell us if you screwed up. Also, is there someone else we can blame? 

Congress is jumping on the Secret Service’s Butler debacle, asserting its own right to investigate the assassination bid.

House Oversight Committee Chairman James Comer has directed Secret Service chief Kimberly Cheatle to “preserve all documents” tied to the Trump shooting, and Cheatle could take a high-profile hammering before Comer’s committee next week.

Meanwhile, Rep. Lance Gooden (R-Tex.), citing a lack of “faith in the current FBI leadership,” is calling for a select House committee to look into the Secret Service’s mess, joined by Rep. Jim Banks (R-In.), while Sens. Rand Paul (R-Ky.) and Gary Peters (D-Mich.) announced plans for a bipartisan Senate investigation.

Yet the White House is telling Americans to blindly trust an agency whose acronym is now widely presumed to stand for Following Biden’s Instructions. 

This is the same FBI that kowtowed when Team Biden wanted parents who protested school closures and mask mandates at school board meetings to be categorized as terrorist threats.

An FBI analysis in 2023 recommended targeting conservative Catholics as potential violent extremists, citing nine “radical-traditionalist Catholic” organizations as “hate groups.” Twenty state attorneys general denounced the Bureau for effectively claiming that it could “distinguish the bad Catholics from the good ones” based on their “preference for ‘the Traditional Latin Mass and pre-Vatican teachings.’”

It’s the same agency that authorized the “use of deadly force” by FBI agents conducting a massively publicized raid on Trump’s Mar-a-Lago residence in August 2022 in pursuit of government documents. (Federal Judge Aileen Cannon dismissed the charges stemming from the raid on Monday.)

The agency knew early on that Hunter Biden’s laptop was bona fide, but did nothing to debunk the claims by CIA officials and others that it was Russian disinformation prior to the 2020 election.

FBI officials also browbeat Twitter and other social media companies to suppress The Post’s bombshell laptop revelations.

Will the FBI treat the Secret Service like it did Hillary Clinton’s 2016 presidential campaign? The FBI shrugged off the destruction of tens of thousands of subpoenaed emails tied to her breach of federal classified data laws.

Actually, the Secret Service has already been caught destroying far more evidence than did Team Hillary.    

In January 2021, a Secret Service sweep missed a pipe bomb planted outside the Democratic National Committee headquarters shortly before Kamala Harris, then the Vice President-elect, arrived there.

Months later, the agency admitted it had erased the cellphone records “of dozens of high-level officials and agents” — doing so after the evidence was requested by the Department of Homeland Security Inspector General. No Secret Service official faced any penalty for that obstruction.

Is the routine destruction of self-incriminating evidence simply another perk of federal law enforcement?

Biden declared on Sunday, “We don’t yet have any information about the motive of the shooter . . . Let the FBI do their job.”

Like the job done by the FBI after the 2016 Pulse nightclub massacre in Orlando, perhaps? Police repeatedly spoke with shooter Omar Mateen via cellphone throughout the night as he was killing 49 people while pledging allegiance to the Islamic State, saying his homicidal spree was spurred by US bombings in Iraq and Syria.

But the FBI initially expunged the comments from the phone transcripts, enabling President Barack Obama to falsely portray the attack as having been spurred by anti-gay bias.

More recently, the FBI fought in federal court to suppress the crazed writings of Audrey Hale, the transgender mass killer who attacked the Covenant Christian School in Nashville last year.

One leaked excerpt purportedly revealed that Hale loathed the children for their “white privilege.”

Perhaps the best recent example of an independent Biden administration “investigation” is Attorney General Merrick Garland’s willingness to risk jail rather than reveal the audiotape of his boss’ interview with Special Counsel Robert Hur.  

Team Biden will stonewall a congressional subpoena for clearly political reasons — what might they suppress regarding the assassination attempt on Trump?

If Biden’s rhetoric dubbing Trump a neo-Hitler influenced Secret Service decisions on the level of protection the Republican deserved, for example, don’t expect the FBI to reveal it.

So congressional investigations and whistleblower leaks are the best chance to get at the truth of the matter between now and Nov. 5.

Our political system has zero legitimacy to spare these days.

Biden said Sunday that “nothing is more important than [unity] right now.” But a façade of unity is no substitute for the truth.

James Bovard’s latest book is “Last Rights: The Death of American Liberty.”