On a day in July 2017, I hired a former US law enforcement officer to buy an AK-47 assault rifle on my behalf. We were in McAllen, a city that sprawls across the Texas-Mexican border.

On the Mexican side, it is called Reynosa, and it's home to a deadly war between the Gulf and Zeta drug cartels.

We had come to investigate the smuggling of American weapons across the border where they are driving a spiral of violence.

I'd found the AK-47 advertised on a website by an online seller and made contact.

Two days later, it was handed over to my law enforcement contact in broad daylight in the busy car park of a filling station in exchange for $600. Neither seller nor buyer knew the other person's identity.

My contact could have been a Mexican cartel member and "our" AK-47 was in effect untraceable.

This transaction was entirely legal under US laws - the very point that we were trying to make.

Inside Mexico's drug war

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The connection between the easy availability of assault rifles and mass killings such as the Valentine's Day massacre at Florida's Marjory Stoneman Douglas High School or the Las Vegas mass shooting is regularly made.

Barely known is the fact, that US gun laws are having a deadly impact in Mexico, too.

According to a report from the US Government Accountability Office, a staggering 70 percent of all crime guns recovered and traced in Mexico between 2009 and 2014 could be traced back to the United States.

Mexico is witnessing an unprecedented explosion in violence caused by the wars over the drug trade.

In an ideal world, we'd like the sale of automatic weapons to stop. Victor Manuel Saenz, Tamaulipas governor's chief of staff

It had been a relief to reach the safety of McAllen after crossing the border in Reynosa. Two heavily armed police escorts had given us protection and it was made clear that under no circumstances must we stop to film on the Mexican side.

But those who live in Reynosa do so facing an escalating murder rate. In September 2017 alone, over 140 people were killed, up by 167 percent from the year before.

Our escort formed part of the Tamaulipas state police who had offered to show us the effect of gun violence in the relatively "safety" of the state capital Ciudad Victoria.

This border state has the highest number of disappeared people in Mexico and the hills around the state capital are still home to the cartels who make it too dangerous for security forces to start proper searches for dead bodies.

Captain Edgar Vallejo Arosa and his team told us that the cartels often had more weapons and ammunition than they did.

They showed us houses that had seen gun battles, but it was the visit to the state prison that proved most memorable: Barely two months earlier, a massive riot had broken out after corrupt officers had allegedly helped smuggle in weapons.

Vallejo Arosa told us prisoners shot at officers with long guns and had a supply of around 6,000 bullets. Three officers from his team died before the prison was retaken.

"Unfortunately, we know that there are [still] many guns inside [the prison] but we don't know where they are hidden," he said.

The Tamaulipas governor's chief of staff, Victor Manuel Saenz, made the connection with guns smuggled into Mexico from the US. He said weak US gun laws carried some blame but that requests to introduce stricter controls on the sale of automatic weapons in the US had fallen on deaf ears.

"In an ideal world, we'd like the sale of automatic weapons to stop. We would like these sales to be limited to sport guns, for personal defence. But if they will carry on selling automatic weapons, these should be very stringently controlled," he said.

Once we were in Texas, we found that US federal investigators agreed. Nicole Strong and Frank Ortega, agents for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) explained that they, too, face a problem of legislation: the US lacks a firearms trafficking statute meaning there is no law under which the trafficking of firearms to be prosecuted.

The firearms trafficking statute ... would allow us to go not just after the straw purchaser, but the entire network of people that are getting these arms to arm the cartel. Nicole Strong, ATF

An Obama-era gun control regulation under fire

One of the few legal tools that the ATF has to spot potential traffickers is an Obama-era reporting requirement for multiple purchases of assault rifles. This obliges a licensed firearms dealer in a US border state to report any buyer who purchases several semi-automatic rifles of a particular type within a short period of time.

Not much, one might think, but even this reporting requirement is under fire. A handful of congressmen are using a bill to try and cancel it, and with President Donald Trump at the helm, they may well succeed.

None of this, however, appeared to concern the unidentified man who sold us the AK-47 in the car park of the filling station.

On his website, he had at least four other rifles for sale, but as long as he can credibly claim that he is a private seller rather than a commercial one, he can trade virtually outside the limits of the law and sell them to anyone without asking who they are.

Which leaves the untraceable AK-47 that we bought from him.

Much to the chagrin of those around me, I ended up destroying it with a grinder.

A week after patrolling with the officers from the Tamaulipas State Police, who are targeted with these kinds of rifles every day, it was an extremely gratifying thing to do.