He did it for years. And only a tip from a legitimate gun parts supplier helped U.S. law enforcement authorities catch him. 

Evidence photos from the U.S. District Court case illustrate the sophisticated operation run out of house in Nuevo Laredo for years, right across the border from Laredo, Texas. 

It's where retired ATF Special Agent Edwin Starr tells CBS News that Andrew Scott Pierson, of Jay, Oklahoma, smuggled ghost gun parts and set up a gun manufacturing operation to supply two different Mexican drug cartels with weapons. 

"He was very slick. He had multiple identities," said Starr, who helped break the case. 

"When his residence was finally searched by Mexican police, they found passports... from countries as far away as Lithuania," Starr said. "He had multiple identities he used in Mexico, voter ID cards, birth certificates. He had multiple identities in states in the United States." 

To solve the case, took years and many agents not only from the Bureau of Alcohol, Tobacco, Firearms and Explosives, but a dozen other agencies including the FBI, State Department and even the U.S. Postal Service. 

"We had a very complex case and we had a lot of agencies participating," said Assistant U.S. Attorney Anne Gardner, who prosecuted the case. "The amount of firearms for which he assisted the cartels in using, fixing, making and trafficking were responsible for hundreds of deaths." 

In 2021, Pierson pleaded guilty in U.S. District Court for the Eastern District of Arkansas to violating the U.S. Arms Export Control Act. 

Pierson, who is 48, is currently serving a 12-year sentence at a federal prison in Texarkana, Texas. 

But the Pierson case isn't the only one where U.S. citizens have been found to help arm Mexican drug cartels. 

Gun trafficking from the U.S. to Mexico

In fact, most firearms found at violent crime scenes in Mexico originated in the United States, according to a recent report by the investigative arm of Congress, which confirms what a CBS Reports documentary recently uncovered. 

In its September 2023 report to Congress, the Government Accountability Office, or GAO, found that while the U.S. has sent more than $3 billion to Mexico since 2008 to fight drug and gun trafficking, the U.S. government can't demonstrate that the money has been spent effectively. 

The GAO reported "about 70 percent of firearms seized in Mexico between 2014 and 2018 … originated in the U.S." 

"It has nothing to do with American citizens owning firearms," said Starr. "It has nothing to do with the Second Amendment. It has everything to do with you cannot export firearms to cartels." 

In the documentary titled "Arming Cartels: Inside the Mexican-American Gunrunning Networks," CBS News talked to cartel members about their ability to get firearms — some of them military grade — on demand to protect their turf and perpetuate their drug trade. 

CBS News uncovered how the gun pipeline works: When narcos want guns they activate a "phone tree" and call accomplices who live across the United States. Those U.S. residents are paid to buy weapons and ammo, then illegally pass them off to brokers. Couriers pick up those guns and then drive them into Mexico and the hands of cartels.

"It is illegal to export firearm components," said Starr. "United States citizens that are doing that need to be prosecuted. They need to be prosecuted to stop these networks from supplying firearms to the cartels." 

U.S. Justice Department officials told CBS News part of the problem lies in how easy it is under current law for guns to be exported — shipped out of the United States, — as compared to how much harder it is to import those same guns. 

"I think there needs to be stricter controls" for exports, said Gardner. "It doesn't seem to be that difficult to drive parts and guns over the border." 

This map, obtained through intelligence sources exclusively by CBS News, shows bright red dots wherever a gun that was purchased was traced directly to cartel violence in Mexico. It shows firearms purchases across the United States that then ended up in the hands of various Mexican cartels. 

Americans caught in the crossfire

The implications affect not only Mexico's battle against cartel violence but American citizens, as well. 

One example is the case of four friends from South Carolina who were caught in the middle of what officials described as a cartel shootout in Matamoros, Mexico, on March 7. 2023. Latavia "Tay" McGee, Eric Williams, Zindell Brown and Shaeed Woodard had traveled to Mexico so one of them could get cosmetic surgery During the shootout, Brown and Woodard were killed and McGee and Williams were kidnapped. 

U.S. law enforcement agents traced one of the guns used by the cartel — a 5.56 caliber semi-automatic pistol — to a U.S. citizen, Roberto Lugardo Moreno, Jr., who was indicted in April 2023. 

Moreno was charged with making false statements when he bought the gun. U.S. prosecutors said Moreno acted as a straw buyer when he purchased the gun at a Brownsville, Texas, pawn shop for the express purpose of shipping it to Mexico to supply the cartel. 

On May 17, Moreno pleaded guilty in U.S. District Court for the Southern District of Texas to smuggling the gun into Mexico. He is now awaiting sentencing, which is scheduled for October 11. 

Starr told CBS News more needs to be done. 

"You're going to have to ask higher government officials that are elected, not appointed, 'Why can't we stop the export of firearms,'" he said. "'Why can't we stop the illegal export?'"