Enrique, wearing braces on his teeth and a black cap, talks quietly and carefully as he describes how he 'cooks' fentanyl.

The materials are bought from China and Germany over the internet, delivered to his home town in northern Mexico and prepared into a block of powder to send north, feeding America's insatiable demand for synthetic opioids.

For the last 50 years cocaine and heroin have been the principle exports driving the growth of Cartels and their reign of terror in Mexico and across the border into the US.

But now, fueled by the opioid crisis that has swept across America, the gangs here in mexico have a new - and much simpler - way of making money.

"Clients in the US are asking for fentanyl right now. There is a lot of opium paste in the market because not that many people are buying it anymore. They’re buying fentanyl instead," Enrique says.

In an attempt to justify calling a national emergency to build his wall across the Mexican border President Trump has himself mentioned the fentanyl "invasion".