This just might be the smoking gun we’ve been waiting for to break the festering “Fast and Furious” gun-running scandal wide open: the Department of Alcohol, Tobacco, Firearms and Explosives apparently ordered one of its own agents to purchase firearms with taxpayer money, and sell them directly to a Mexican drug cartel.

Let that sink in: After months of pretending that “Fast and Furious” was a botched surveillance operation of illegal gun-running spearheaded by the ATF and the US attorney’s office in Phoenix, it turns out that the government itself was selling guns to the bad guys.

Agent John Dodson was ordered to buy four Draco pistols for cash and even got a letter from his supervisor, David Voth, authorizing a federally licensed gun dealer to sell him the guns without bothering about the necessary paperwork.

“Please accept this letter in lieu of completing an ATF Form 4473 for the purchase of four (4) CAI, Model Draco, 7.62×39 mm pistols, by Special Agent John Dodson,” read the June 1, 2010, letter. “These aforementioned pistols will be used by Special Agent Dodson in furtherance of performance of his official duties.”

On orders, Dodson then sold the guns to known criminals, who first stashed them away and then — deliberately unhindered by the ATF or any other agency — whisked them off to Mexico.

People were killed with Fast and Furious weapons, including at least two American agents and hundreds of Mexicans. And the taxpayers picked up the bill.

So where’s the outrage?

There’s none from the feds. Attorney General Eric Holder has consistently stonewalled Rep. Darrell Issa, Sen. Chuck Grassley and other congressional investigators.

In a constantly evolving set of lies, Holder has denied knowing anything about Fast and Furious while at the same time withholding documents from the House and Senate committees looking into the mess while muzzling some witnesses and transferring others.

Justice calls the allegations about Dodson’s operation “false.” But Grassley says that’s “a lie,” as he told Greta van Susteren the other day. “The ATF ordered this ATF agent to purchase these guns and in turn sell them, and supposedly track them,” he said. “But he was a lone wolf in the operation — they wouldn’t give him any help for 24-hour surveillance.”

So now the wheels have come off the official explanation for Fast and Furious. Of course, that explanation never made much sense in the first place.

For one thing, the ATF had no authority to track the guns once they were in Mexico; for another, nobody bothered to inform the Mexicans of this intrusion on their national sovereignty.

Further, we now know that a host of federal agencies (including the ATF, the FBI and IRS, Immigration and Customs Enforcement, the Drug Enforcement Administration and, very probably, top officials at the Department of Homeland Security) were all in the loop at various levels, as was the White House.

So calling “Fast and Furious” a cockamamie operation gone wrong just isn’t going to cut it anymore.

There are two possible explanations. The first is that the anti-gun Obama administration deliberately wanted American guns planted in Mexico in order to demonize American firearms dealers and gun owners. The operation was manufacturing “evidence” for the president’s false claim that we’re to blame for the appalling levels of Mexican drug-war violence.

If this is true, then Holder & Co. have got to go — and the trail needs to be followed no matter where it leads. For the federal government to seek to frame its own citizens is unconscionable.

A second notion is that the CIA was behind the whole thing, which accounts for all the desperate wagon-circling. Under this theory, the Agency feared the los Zetas drug cartel was becoming too powerful and might even mount a coup against the Mexican government. So some 2,000 weapons costing more than $1.25 million were deliberately channeled to the rival Sinaloa cartel, which operates along the American border, to keep the Zetas in check.

Of course, there’s a third explanation — that both scenarios are true, and that those in charge of Fast and Furious saw an opportunity to shoot two birds with one Romanian-made AK Draco pistol.

Time for a special prosecutor, who’s both fast and furious.

Michael Walsh’s new spy thriller, “Shock Warning,” hits stores this week.