Nearly 120lbs (54kg) of fentanyl, a powerful synthetic painkiller, has been seized by police in Nebraska - one of the largest busts in US history.
The drugs, seized last month, could kill over 26 million people, according to estimates by the US Drug Enforcement Administration (DEA).
Police found the fentanyl in a fake compartment of a lorry. The driver and a passenger were arrested.
Fentanyl is a synthetic opioid that is 30-50 times more potent than heroin.
It was the largest seizure of fentanyl in state history, Nebraska State Patrol said in a Twitter post on Thursday.
Police stopped Felipe Genao-Minaya, 46, and his 52-year-old passenger Nelson Nunez, both of New Jersey, on 26 April after spotting the pair driving on the shoulder of a motorway near the city of Kearney.
During the stop, a trooper "became suspicious of criminal activity" and searched the lorry to discover 42 foil-wrapped packages containing 118lbs (53kg) of fentanyl.
State troopers initially thought they had discovered a mix of narcotics and cocaine, but in Thursday's announcement officials said testing proved the drugs were "entirely fentanyl".
The two men were arrested on suspicion of possession of a controlled substance with intent to deliver and no Drug Tax stamp, according to a Nebraska State Patrol statement earlier this month.
They are being held in county jail on a $100,000 (£74,000) bond.
What is fentanyl?
Fentanyl is a synthetic opioid that appears as a white powder, similar in size to grains of salt.
Just 2mg of fentanyl - or a few grains of table salt - is a lethal dosage for most people, and even exposure can cause a fatal reaction, according to the DEA.
According to DEA estimates, the 118lbs could kill about 26 million people.
Like heroin and other opioids, fentanyl causes drowsiness, nausea and confusion, and overdoses can result in respiratory failure and death.
In the US, it is approved as an anaesthetic and for pain relief, but because of its high profit margin for traffickers, it has become a large part of America's opioid crisis.
The Center for Disease Control (CDC) reported that between 2015 and 2016, the rate of drug overdose deaths involving synthetic opioids such as fentanyl had doubled.
Medical examiners ruled in 2016 that US musician Prince died from an accidental overdose of the drug.