Internal government documents disclosed to a team of Senate investigators offer new details about a secret intelligence project that was successfully dismantling U.S. weapons smuggling networks into Mexico until it was abruptly shut down without explanation in fiscal year 2022.

Whistleblower disclosures made to Iowa Sen. Chuck Grassley's office confirmed and expanded upon the existence and success of Project Thor, first reported by CBS News last September. 

The new documents, exclusively obtained by CBS News, were sent by Grassley on Thursday to nine federal law enforcement agencies within the Departments of Justice, Homeland Security, State and Postal Service. They accompanied a set of sternly-worded letters demanding that each department disclose details about Thor's operations — the existence of which the Biden administration has yet to publicly acknowledge — as well as all current efforts to target cartel firearms trafficking networks operating in the U.S. and neighboring countries.

"The American people deserve to know more about the Biden administration's strategy … to target cartel firearms networks in the U.S.," wrote Grassley, questioning exactly how committed the U.S. government has been to stem the flow of American weapons into the hands of cartels.

The FBI and State Department declined to comment. The Postal Service confirmed that they were reviewing the letter and "will be providing a response directly to the Senator."  In an email, Justice Department spokesman Peter Carr also acknowledged receiving Grassley's letter. 

"The Justice Department has maintained an ongoing, high-level, and extremely productive dialogue with the Government of Mexico on cooperative efforts to stem cross-border firearms trafficking. We are focused on what we do best: investigating criminal activities, enterprises, and conspiracies, and bringing prosecutions," Carr replied, noting they have charged more than 300 people since congress passed the 2022 Bipartisan Safer Communities Act, which included a new firearms trafficking statute.

Grassley's newly released documents make public, for the first time, internal charts detailing how intricate networks of traffickers across the U.S. smuggle firearms and ammunition across the southwest border into Mexico and the hands of drug cartels, including depictions of actual narco weapons trafficking routes. 

This follows a CBS News investigation that revealed that Mexican drug cartels have been smuggling a vast arsenal of military-grade weapons out of the U.S. with the help of American citizens, including belt-fed miniguns and grenade launchers. Intelligence documents and interviews with half a dozen current and former officials showed that the U.S. government has known this for years but, sources said, it's done little to stop these weapons trafficking networks inside the United States, which move up to a million firearms across the border annually.

The latest disclosures are part of an inquiry launched by Grassley in October, which sought to uncover why the federal government canceled an interagency intelligence effort at the same time that it praised its success. 

In response to three requests for oversight information, the Bureau of Alcohol, Tobacco and Firearms, or ATF, provided investigators with more than 700 pages of heavily redacted government documents exclusively reviewed by CBS News. 

"ATF is steadfast in its commitment to identify, disrupt, and dismantle firearms trafficking networks — including but not limited to those who wish to supply illegal firearms to Mexican cartels and other transnational criminal and terrorist organizations," Ann Vallandingham, ATF Assistant Director for Public and Government Affairs, wrote on Jan. 5. While those documents detailed some of the internal workings of Project Thor, they did not fully respond to investigators' questions. 

"Until Thor, no one was focusing on the problem," according to a law enforcement sensitive PowerPoint presentation released by Grassley on Thursday, estimating cartels smuggled up to $503 million-worth of weapons and ammunition annually. The documents revealed that, while it was operational, Project Thor "supported 76 cartel trafficking cases from 2018-2020" and "identified most of the major CJNG weapons trafficking networks," referring to one of Mexico's biggest and deadliest cartels.

Justice Department officials have repeatedly declined to comment on Project Thor or its findings, claiming that they were "not familiar" with it — an admission that Grassley called "surprising" since it appeared inconsistent with internal agency correspondence provided to his team of investigators. For example, one document noted that Project Thor involved more than 16 executive branch agencies including "Main DOJ," DEA, and FBI.

An email sent by the ATF in the summer of 2021 detailed plans to conduct a Project Thor briefing alongside DOJ representatives at the U.S. embassy in Mexico two weeks after the arrival of newly appointed Ambassador Ken Salazar. 

"The briefing that week is still a go," ATF International Affairs Chief Joshua Rusk wrote to Division Chief Ned Dixon, Assistant Special Agent in Charge Will Panoke and a redacted party. A separate email forwarded from ATF headquarters to the prior deputy mission chief and U.S. ambassador to Mexico identified Rebecca McCormack as "our DOJ Trial Attorney and DOJ lead" for Project Thor.

In his letters, Grassley noted that records independently provided to his office supported CBS News' findings about Project Thor and reiterated concerns about the effectiveness of America's current efforts to stop firearms trafficking to Mexico.

"Questions have been raised about law enforcement, intelligence, and diplomatic agencies' commitment to dismantling cartel gunrunning networks operating across the U.S and in neighboring countries," Grassley wrote to law enforcement leadership.

Last month, Grassley privately sent a letter to the U.S. ambassador to Mexico that provided evidence that the U.S. government has been aware that even firearms belonging to American armed forces were in the hands of Mexican narcos. The letter noted his investigation's "shocking finding … that the U.S. government had evidence of severe mishandling of U.S. resources to Mexico for some years. Yet, no corrective action appears to have been taken." In response, a State Department representative told Grassley they shared his concerns and offered to brief him about their progress "in the coming months."

On the opposite side of the aisle, congressional Democrats introduced two bills in late 2023 — the Stop Arming Cartels Act and Disarming Cartels Act — aimed at thwarting America's illicit international weapons trade. Democratic Judiciary Committee Chair Sen. Dick Durbin is preparing to hold a hearing on the issue of gun trafficking from the U.S. to drug cartels called "Damming the Iron River."

In January, the Mexican government demanded a U.S. investigation into American military-grade weapons being trafficked to cartels, as a U.S. appeals court revived a related $10 billion lawsuit against gun manufacturers.

The Biden administration recently reaffirmed its commitment alongside Mexico and Canada to stem the flow of American weapons into the hands of drug cartels. But neither the Justice Department nor ATF have provided evidence to demonstrate that their efforts meaningfully reduced the flow of American firearms to Mexico. U.S. law enforcement seized 1,720 firearms in the first six months of fiscal year 2023, which accounted for less than 1% of all firearms being smuggled across the border, based on estimates by Project Thor and the Mexican government.

Sarah Metz contributed reporting.

E.D. Cauchi

E.D. Cauchi is an investigative journalist covering people and institutions in power for CBS News' longform unit. She previously worked for CNN, NBC News, National Geographic, BBC News, Al Jazeera and others, and co-authored a nonfiction book about U.S. warfare post-9/11.