IN A cavernous warehouse near Los Angeles International Airport, United States Customs and Border Patrol (CBP) officers process six packages per second. As cardboard boxes and manilla envelopes from around the world stream through x-ray machines, officers with handguns tucked into their waistbands scan screens for anomalies in the images. Last summer officers at the warehouse found three live King Cobras coiled into aerated potato-crisp cans. On a recent morning they found nothing creepy or crawly, only bags full of dried orange skins and Chinese meat snacks disguised as candy. Mostly, however, they found drugs: counterfeit Viagra, vials of steroids and small plastic bags full of unidentified white powders.

Officers are particularly worried about one drug: fentanyl. Kevin McAleenan, the acting commissioner of CBP, says the drug is the agency’s priority. A synthetic opioid, fentanyl is 50 times stronger than heroin and 100 times more potent than morphine. The drug is largely behind the increase in America’s drug-overdose death rate. Between June 2015 and June 2017, overdose deaths rose by 34%. During the same period, fatalities linked to synthetic opioids other than methadone, a category dominated by fentanyl, more than tripled from 7,551 to 23,995. On January 10th President Donald Trump signed the INTERDICT Act, a law that will provide CBP with $9m in extra funding to look for fentanyl. On February 1st China will begin restricting two precursors used to synthesise fentanyl, which American officials hope will stem the flow into the country.

The drug has long been used legally to treat cancer pain, but in recent years has flooded into America’s black market, where it is found mixed into other drugs, punched into pills that resemble prescription painkillers or, less commonly, sold on its own. Fentanyl is very profitable for drug-traffickers: a recent Drug Enforcement Administration report estimated that a kilogram of heroin sells for $80,000 on the street, whereas a kilogram of fentanyl can command between $1.28m and $1.92m. So traffickers are highly motivated to push it on their customers. Consumers often use it inadvertently, unaware that it has been stirred into their heroin or that their illicit OxyContin pills are not what they seem.

The Los Angeles warehouse represents one of the front lines in the government’s fight to keep illicit fentanyl out of the country. “We and the warehouse at JFK (New York’s international airport), we’re ground zero,” says Rolando Knight, a veteran CBP officer who supervises the Los Angeles operation. In its illegal form, fentanyl is mostly produced in China.

The drug’s potency means it can be concealed in packets and boxes small enough to be sent by international mail. Fentanyl reaches America in two main ways. Some is posted to Mexico, where traffickers usually mix it with other substances, such as heroin and cocaine, before sneaking it across the border and into the hands of drug-dealers. In other cases, pure fentanyl is sent directly to America, where at some point it must pass through a facility like the one in Los Angeles.

Between October 2016 and September 7th 2017, CBP seized 299lb (136kg) of fentanyl sent through international postal services and private carriers such as FedEx, UPS and DHL. During the same period the agency seized 494lb of the drug on America’s land border with Mexico, but it was often mixed with other substances. The average purity of fentanyl shipped into America by post is over 90%, compared with 7% for that seized on the land borders.

A visit to the Los Angeles warehouse underscores just how manual the process is. On a recent morning, an officer at the Los Angeles warehouse squinted as he noticed something fishy about a package passing through his x-ray machine. He punched a red button to stop the conveyor belt and picked up the package in question: he could tell that it had come from China, but it had no return address. He slashed the tape open with a box-cutter and found a paint-roller with dime bags of white powder taped inside.

The bags were passed to Jaime Pimentel, another CBP employee whose job is to test the substance. Behind him on a shelf sat a box of Naloxone, an opioid-overdose antidote, in case Mr Pimentel accidentally ingests fentanyl or another opioid. He pressed a machine called a TruNarc onto a plastic bag full of powder and waited for a reading. It came back inconclusive. Then he used a metal scooper to sprinkle a small sample of the powder into a more sophisticated device called the Gemini, which resembles a clunky Gameboy. It also failed to match the substance to any of the 21 drugs in its database, so the powder had to be sent to a nearby forensic lab for further testing, which could take a few weeks. “The federal government has responded impressively quick to the fentanyl threat, which really didn’t escalate until 2015,” Mr Knight muses. “But we’re still walking—crawling really. It’s so hard to seize, that when we do we almost have to say ‘Wow! Good job!’”