It would take only 118 pounds of fentanyl to kill 25 million people.

That’s how much of the powerful opioid painkiller Nebraska State Trooper Sam Mortensen found in April when he stopped a truck marked “U.S. Mail” swerving onto the shoulder along Interstate 80.

Rolling up the trailer door revealed an empty hold. But just below a refrigeration unit, behind a plastic panel secured with mismatched bolts, Mortensen found 42 brick-shaped packages, weighing 54 kilograms, full of fentanyl. The drug is so potent that even a small amount — the equivalent of a few grains of salt — can be lethal.

“Is that even believable? Can you even imagine?” President Donald Trump said in October when Mortensen was honored at the White House for making one of the largest fentanyl seizures in U.S. history. The truck’s two drivers were arrested. “Trooper Mortensen, that was a job well done.”

Fentanyl has emerged as the most dangerous of a group of drugs blamed for creating a U.S. public health crisis. U.S. deaths linked to fentanyl grew more than 50 percent to 29,406 last year, from 19,413 in 2016, according to the National Institute on Drug Abuse (NIDA). Relatively easy to manufacture, the drug is turning up more on the streets as dealers strive to meet still-enormous demand for opioids in the U.S.

Fentanyl is ever-evolving as suppliers try to avoid detection and still boost the potency of the drug using what are called analogues — essentially chemical cousins.

“There’s never been a drug like fentanyl before,” said Josh Bloom, senior director of chemical and pharmaceutical research at the American Council on Science and Health. “For street drugs, this absolutely destroys anything else in terms of lethality and danger.”

Fentanyl is 50 times more potent than heroin, with which it is often mixed. In its strongest form, called carfentanil, it is used legally as an elephant tranquilizer. Law enforcement officers and first responders have been warned to handle fentanyl with extreme caution; some have fallen seriously ill after getting it on their skin or clothing.

The fatal potential of even glancing contact with fentanyl is a major reason why national security experts are becoming alarmed at the prospect of it being used to sow terror. The drug is “a significant threat to national security,” Michael Morell, former CIA acting director under President Barack Obama, wrote last year. “It is a weapon of mass destruction.”

The use of fentanyl as a weapon isn’t new. In 2002, 50 armed rebels held more than 800 hostages in a crowded theater in Moscow, demanding the withdrawal of Russian forces from Chechnya. After a few days, Russian forces used a gas, reported by state news agency Interfax to be fentanyl, to incapacitate the attackers; more than 100 hostages were also killed.

As a tool of terror, the drug would work best in a closed space, said Daniel Gerstein, a senior policy researcher at Rand Corp., who served as acting undersecretary in the Department of Homeland Security’s Science and Technology Directorate in the Obama administration. Open-air release likely wouldn’t be as effective, as the drug could become too diluted, he said.

If ground-up fentanyl is placed on everyday objects, people could easily put their fingers in their mouths or rub their eyes and have a deadly reaction, said Bloom.

“It doesn’t take much more than a half-competent ­chemist to be able to manufacture it. And it’s cheaper to manufacture than heroin.”

Overdoses are hard to reverse

Containing a fentanyl attack would be difficult for police and emergency medical officials. Overdoses of the drug are hard to reverse with existing formulations of antidotes such as the Narcan nasal spray.

Narcan is carried by many police and paramedics, especially in areas hard-hit by the recent opioid epidemic. But people incapacitated by fentanyl frequently require several doses. Even some police and other emergency officials who have accidentally ingested or absorbed the drug have needed multiple blasts of Narcan to be brought back.

Last year, police officer Chris Green made a traffic stop in East Liverpool, Ohio, and ended up with fentanyl powder on his shirt. After another officer pointed it out, Green brushed it off with his hand. Soon, paramedics were rushing him to the hospital.

“He realizes something ain’t right,” said Police Chief John Lane. “He gets lightheaded.”

Green survived after being given four doses of Narcan. Though skin contact with fentanyl isn’t typically deadly, Green had used sanitizer on his hands, which hastened the absorption of the fentanyl. A video distributed to law enforcement in August by U.S. Customs and Border Protection warns against using hand sanitizer and says those who have touched fentanyl shouldn’t touch their eyes, nose or mouth.

The U.S. Biomedical Advanced Research and Development Authority, known as Barda, is tasked with developing medical countermeasures. In September, it penned a potential $4.6 million contract with Opiant Pharmaceuticals Inc. to produce a reliable single-dose fentanyl antidote.

“Fentanyl-based drugs have been used in conflicts in other countries, so we know it’s possible, and we need to be ready to save lives and protect Americans from potential health security threats,” said Barda Director Rick Bright.