sealed in plastic, then dropped with an oily splash into the petrol tank of the trafficker's nondescript car.

Jay, not his real name, had crossed earlier from the US to this cartel-run safe house on the Mexican side of the border. The house looks like any other in this neighbourhood. We are told to drive in quickly and an iron gate closes firmly behind us. They don't cook the drug here, but still they are wary of attracting attention. The men all speak in hushed voices and work quickly.

Their lethal business has become the centre of a dispute causing shockwaves in the global economy after the White House used fentanyl smuggling through US borders as a key justification for raising tariffs. US President Donald Trump has also vowed to "wage war" on the drug cartels.

The BBC gained rare access to a cartel's operation along the border and travelled to the US to meet their ultimate customers, to see if the international row was doing anything to halt the illegal flow of narcotics.

The men we meet at the safe house are foot-soldiers of a well-known cartel. Two of them loading the car admit to fleeting moments of remorse. But when I ask the man packing the drugs into the fuel tank if he feels guilty about the deaths the pills cause, he sniggers. "We have family too, of course we feel guilty. But if I stop, it's going to continue. It's not my problem," he tells me with a shrug.