How do cartels get drugs into the US?
A recent report from the US Drug Enforcement Administration (DEA) said that drug overdoses kill more people in the US than car accidents or guns. Some deaths are through prescription drugs, others by cocaine or, increasingly, heroin.
Many of these drugs are smuggled in large volumes by drug cartels from the fields of South America past the thousands of patrol officers who guard the US border.
Four experts spoke to the BBC World Service Inquiry programme about how the smugglers do it.
Rear Admiral Christopher Tomney: Tackling the priority networks
Rear Admiral Christopher Tomney is director of Joint Interagency Task Force South for the US Coast Guard.
"We cover over 40 million square nautical miles, and reach well out into the Atlantic, throughout north, central and south America as well as all the way out into the eastern Pacific.
"The number one drug we see moving is cocaine. Last year we were able to successfully take out of the pipeline 191 metric tonnes of cocaine. Around 20 to 25% of cocaine around the globe is interdicted.
"The cartels are very innovative. Due to their large profits, they have a lot of money they can throw at technology.
"In the early days of this task force - and we've been around for 26 years - we saw much higher movement using non-commercial aircraft to fly the drugs northwards.
"[Now] well over 95% of the drugs are moving on the water via container ships, non-commercial vessels, pleasure boats, sail boats, fishing boats. They also have fast boats which try to outrun our law enforcement assets.
"We've seen growing use of self-propelled semi-submersibles (SPSSs) - low-profile vessels made out of marine-grade plywood [and] fibreglass with commercial engines. The smugglers spend up to a $1m (£665,000) to build one of these SPSSs for what is often just a one-way voyage.
"With my international and area agency coalition, I have more situational awareness of the movement of drugs than I have operational assets. So to me, it is a resource problem. We're being very effective with the capabilities we have, but I could use more ships and more aircraft.
"Just because we are unable to stop all the drugs moving doesn't mean we are losing this fight. My goal is to go after the priority networks who rival the capabilities of nation states. We need to knock down these cartels. Until we do that we have much more work to do."
Lt Commander Devon Brennan: No shortage of ingenuity
Lieutenant Commander Devon Brennan oversees counter-narcotics policies for the US Coast Guard. In September, he monitored an operation which led to the largest seizure of drugs from an SPSS.
"Working with maritime patrol aircraft, the US Coast Guard cutter Bertholf was able to locate a vessel off the coast of Guatemala. As soon as you hear those words, your blood pressure comes up a little bit, and you start to get excited.
"We launched the helicopter - she had, to narrow down the exact area [the vessel] was in - and then launched multiple small boats to sneak up behind the vessel and then we were able to surprise them.
"We physically recovered over 6,800kg of cocaine from the vessel, but there was also an estimated additional 1,200kg on board the vessel. We left it on board because these vessels are very unstable once you remove all the weight in them. While we were towing the vessel it took on water and sank.
"Looking at the vessel, there'll be about a foot sticking above the water, painted various shades of blue to blend in with the water, so it is incredibly hard to detect even a few hundred feet away from you.
"It is purpose-built to transport narcotics, so the overwhelming majority of the space is for drugs. What's left for the crew - generally four people - is very cramped, maybe the size of a bed. No toilet or cooking facilities. They're also exposed to the diesel engines that operate these things.
"The fact that they are so low to the waterlines and filled with so much narcotics, means there is the chance, any time, that the vessels could sink and capsize, and they'd be trapped inside. It is an incredibly dangerous venture, besides the risk of being caught and prosecuted.
"There is no shortage of ingenuity in the drug trafficking organisations. They are clearly trying to develop a fully-submersible vessel, and that would be a game changer for us."
Eric Feldman: Tackling the cross-border tunnels
Eric Feldman is Assistant Special Agent in Charge for Homeland Security Investigations and head of the Tunnel Task Force.
"The cartels are always looking for a guaranteed way to cross narcotics into the US, and a sophisticated tunnel can allow them to cross large-scale loads. Last month we interdicted a tunnel in San Diego and seized more than 10 tonnes of marijuana.
"Compare that to sending four guys across with 30 pounds of marijuana on their backs, who have to navigate the mountains, evade Border Patrols and arrange delivery, and you can see the difference in profit.
"This particular tunnel was over 600 metres long at 40 feet under the surface. It had lighting, a mechanism to circulate the air and a railway system so they could load the product in Mexico and run it down the rails to the exit point. They used a pulley system to pull the narcotics up into the warehouse.
"We've seen a variety of clever attempts to disguise entrances, everything from pool tables, electric panels, hydraulic bathtubs, elevators. We saw an exit point in the US where each time they came through the opening, they filled it with concrete, painted it and re-carpeted it. That's attention to detail.
"We have had success in locating sophisticated cross-border tunnels, but to put it into perspective, since 2006 we have only found 11. We consider [the tunnels] a national security threat, and each time they complete one, our hope is that we identify it and take it out of business before it can be used."
Rodrigo Canales: Don't underestimate organisational brilliance
Rodrigo Canales is an associate professor at the Yale School of Management.
"Our traditional framing is that drug dealers are crazy, ruthless criminals, but I realised they were playing a much more sophisticated sport.
"Because drugs are illegal and physical in nature, the only way you can reliably guarantee to your customers that you're going to deliver, is if you control the territory through which you transport them.
"The way you control competitors is through violence. To control the authorities you use what in Mexico is called 'the law of silver or lead'; meaning if you're the local authority, do you want to earn a lot of money? Or do you want me to kill you?
"And if you want stability in your territory, it's very useful to have local people on your side, or at least not openly against you. In many cases, the most sophisticated organisations establish good relationships with local communities. They help with other types of crime. They invest in the community, and have sophisticated communication strategies to provide a narrative for why they do what they do.
"Organisations which operate effectively have a well-defined structure, culture and set of values. They have YouTube channels. They use social media both to recruit people and communicate with each other.
"The Sinaloa Federation - which is often cited as the most powerful of the Mexican organisations - is quite disciplined about not engaging in other crime, which allows them to retain good relationships with locals. Their brand is professional multinational drug organisation.
"At the other extreme we have Los Zetas. Born as an enforcement arm to the Gulf cartel, the first cohort used to be an elite corps within the military. They maintained the military structure and chain of command. As Los Zetas grew, they overtook the Gulf cartel.
"They've developed a brand of being the most ruthless, violent and feared of the cartels. Once they dominate a territory, they take over all criminal activities within that territory, including extortion and prostitution.
"If you think about the way drug policy has operated in the world and especially in Mexico, we have created an evolutionary environment where only the most ruthless, violent and sophisticated organisations can survive. The ones that are not as sophisticated just get weeded out, either because the government or a competitor overpowers them.
"If we don't take a structural and complete approach, then they're always going to be ahead of us - we're just never going to catch up."