MEXICO CITY — When a former Oklahoma Narcotics Bureau agent, Frank Reyes, pleaded guilty not long ago to running guns, it opened a window on the way Mexican drug cartels fill their arsenals.

The Americans buying guns for Mexican gangsters as part of Reyes’ ring are a gamut of college students, jobless men, gun show employees, city workers — in short, the guy next door.

The “”iron river of guns,”” as the flow of assault rifles into Mexico has been called, is an irritant in U.S.-Mexican relations.

In Mexico, it’s illegal to buy most types of weapons. Drug cartels, with their long tentacles into the United States, use their U.S. networks to snap up assault rifles and other weapons from gun shops and shows and bring them back over the border.

What to do about it is a source of friction within the U.S. government. Earlier this month, the inspector general’s office within the Justice Department issued a report that was critical of the Bureau of Alcohol, Tobacco, Firearms and Explosives for failing to slow the flow of illegal weapons to Mexico.

“”We found that ATF does not systematically and consistently exchange intelligence with its Mexican and some U.S. partner agencies,”” the Nov. 9 report says.

While the ATF clearly has shortcomings, it chalked up a success in breaking apart a ring led by Reyes, a stocky 5-foot-8 former state narcotics agent living in a high-rise apartment building in Oklahoma City near the memorial to the 1995 terrorist bombing of the Murrah Federal Building, which killed 168 people.

Earlier in the year, ATF agents in Texas had intercepted vehicles carrying 28 assault rifles bound for Mexico, and as they traced the serial numbers and purchase sites of the rifles the paper trail led to Oklahoma City and 29-year-old Reyes.

According to an affidavit by ATF agent Michael D. Randall, federal officers began interviewing the “”straw buyers”” — or front-end buyers — of the weapons, who were listed on the Firearms Transaction Record, which gun shop owners are required to obtain upon sale.

The purchaser of some of the weapons was a 25-year-old student, Jorge Blanco, who lived in Stillwater, home of Oklahoma State University. Blanco was a joint owner with Reyes of Kiko’s Hookah Lounge, a local bar. Blanco told ATF agents that Reyes sent him out four times in April to buy AK-47s and Chinese-made SKS assault rifles.

Other buyers for Reyes included a man he met at a coin laundry, a Stillwater city employee, a police officer of the Absentee Shawnee tribe and numerous others.

“”Reyes is always looking for assault rifles and even .50-caliber rifles,”” the police officer, Josh Bufford, who later collaborated with the investigation, told the ATF.

One morning in June, ATF agents watched as Reyes spent $11,000 to buy a Barrett .50-caliber sniper rifle and a Century semi-automatic assault rifle. Unbeknown to Reyes, the gun shop owner was cooperating with the feds, and an electronic tracking device was inserted in the package.

Reyes lifted the gun case into his 2007 Chevrolet Tahoe and headed south down Interstate 35 into Texas, eventually transferring the weapons to a driver in a black BMW convertible. Reyes headed home, but the weapons went south.

Six days later, the ATF alerted Texas law enforcement to stop a two-vehicle caravan near the Eagle Pass border crossing. One of the vehicles, a Ford Explorer, contained the Barrett sniper rifle and 13 assault rifles. An accompanying Saturn contained 30 assault rifles.

On Sept. 29, Reyes pleaded guilty to one count of conspiracy and one count of transferring firearms. He’s to be sentenced in December or January.

It isn’t publicly known for whom Reyes worked in Mexico. It is known that smuggled guns are used in horrendous crimes. Federal agents seized 336 firearms from a Houston smuggling ring in 2006-07, and tracked the weapons to 57 deaths, including those of 18 Mexican law enforcement officers, the Nov. 9 inspector general’s report says.

Prosecuting weapons smuggling rings isn’t easy. While prosecutors can apply any of 75 statutes, such as conspiracy, against gun smugglers, there’s no single law against firearms trafficking, the report says.

Moreover, U.S. gun shops aren’t required to report to the ATF when they sell several assault weapons of the sort cartels prefer in a single purchase, it says, recommending that the bureau redress that issue.

Arturo Sarukhan, the Mexican ambassador to Washington, appealed Nov. 10 to the National Rifle Association to recognize how current laws “”pose a significant threat to Mexico’s security.”” He urged the powerful gun lobby to help legislators enact tougher regulations.

“”This would be a win-win for the NRA,”” Sarukhan said in New York. “”They ensure they are not being criticized for … either complicity, overtly or covertly allowing guns to go into the hands of drug traffickers who then cross them over the border into Mexico.””

The NRA dismissed any need for tougher gun laws, and said U.S. gun shops and shows weren’t used to stock cartel arsenals.

“”These are multibillion-dollar business operations. They are not getting their guns one piece at a time at gun shows in the United States,”” spokesman Andrew Arulanandam said.

He said U.S. legislators should take no role “”to combat this tragic problem. Simply, this is a Mexican problem.””