Washington — A top senator on Capitol Hill is pushing the Biden administration to provide more information on the steps it is taking to stem the flow of firearms from the U.S. into Mexico following a CBS Reports investigation that exposed how Americans are helping Mexican drug cartels smuggle weapons across the southern border.

Sen. Chuck Grassley, a Republican from Iowa, sent a letter to the head of the Bureau of Alcohol, Tobacco, Firearms and Explosives on Tuesday requesting all records relating to Project Thor and a briefing on the program, which was an interagency initiative launched in 2018 aimed at identifying and dismantling the supply chains across the U.S. that provide weapons to Mexican drug cartels. The effort was denied funding for fiscal year 2022 by ATF, CBS News found.

In his letter to ATF Director Steven Dettelbach, Grassley accused the bureau of directing resources away from targeting criminal actors to focus on law-abiding gun owners and sellers instead, citing its revocation of federal firearms licenses and a proposed rule that broadens the definition of who is required to become a federal firearms licensee. 

"Instead of encroaching on the constitutional rights of law-abiding Americans, ATF should dedicate its apparently limited resources to operations that target violent criminals and cartel firearms trafficking networks in the United States," he wrote in the letter, which cited CBS News' findings.

The senator is seeking a list of all government agencies involved in Project Thor and a description of their roles, information about the funding dedicated to the operation and why it was discontinued, and details about how ATF works with other federal law enforcement agencies and the Mexican government to share information about guns bought in the U.S. that were trafficked to cartels in Mexico. 

Grassley, the top Republican on the Senate Budget Committee, has also directed congressional investigators to expand an inquiry on federal efforts to combat Mexican cartels to look into the smuggling networks operating inside the country, which rely in large part on Americans who are paid to buy weapons from gun stores and online dealers. Grassley was one of the lead Senate investigators of the botched ATF program Operation Fast and Furious, during which agents allowed guns to be illegally trafficked into Mexico to identify and track their whereabouts.

In addition to Grassley's push for information, a group of six Democratic senators, led by Judiciary Committee Chairman Dick Durbin, introduced legislation last month that aims to curb firearms trafficking from the U.S. into Mexico.

The measure, called the Stop Arming Cartels Act, followed publication of the CBS Reports documentary that found American citizens have been helping Mexican drug cartels smuggle an arsenal of weapons, including military-grade firearms, out of the U.S. Once the firearms are bought by straw purchasers, they are moved across the southern border through brokers and couriers. 

U.S. intelligence documents and interviews with current and former federal officials revealed that the U.S. government has known about the firearms trafficking by drug cartels for years, but has done little to stop the networks operating in the U.S. CBS News reported that these networks move up to 1 million firearms across the southern border each year. 

U.S. officials have acknowledged how the flow of guns out of the U.S. and into Mexico has enabled drug cartels behind the nation's deadly fentanyl epidemic to protect their operations and successfully outgun Mexican law enforcement.

To disrupt the flow of drugs and guns between the U.S. and Mexico, and take down trafficking networks, senior Biden administration officials said they are boosting collaboration with Mexican authorities.

Secretary of State Antony Blinken, Secretary of Homeland Security Alejandro Mayorkas and Attorney General Merrick Garland traveled to Mexico this month to meet with their counterparts for discussions that focused in part on the movement of illicit firearms and synthetic drugs.

"We in the United States well understand the dangers of the military-grade weapons that are being trafficked to Mexico. They are a serious danger to the United States and a serious danger to Mexico because they defend the cartels," Garland said during a press conference last week. "So we will do everything in our power to stop the unlawful trafficking of weapons to the drug traffickers as part of our fight to break up every link of the chain of the drug traffickers."

The attorney general said the Justice Department has been employing new authorities granted by Congress to prosecute gun traffickers and seize illegal firearms, and highlighted the recent arrests of seven people in Texas who were charged for buying more than 100 guns that were trafficked into Mexico.

Garland also pointed to the work of Operation Southbound, a project that involves nine multi-agency firearms task forces that aim to disrupt gun trafficking.

Mayorkas, too, said the Department of Homeland Security will be sharing with Mexican partners a monthly report on the movement and intended movement of guns bound for Mexico in an effort to facilitate joint operations and investigations. Through Operation Without a Trace, a Homeland Security project to combat illegal trafficking of guns from the U.S. into Mexico, interdiction of firearms has risen more than 44% in the past year, the secretary said.