COLUMBUS, Ohio — Federal agents arrested four people Monday and seized enough fentanyl to kill nearly 5 million people.

Tomas Sandoval, Alvaro Gasca-Cardoso, Salatiel Ramos-Rajos and Alexis Zazueta-Soto have been charged with conspiracy to possess with intent to distribute fentanyl, methamphetamine and heroin.

They were in federal court Monday afternoon.

According to U.S. Attorney Ben Glassman, investigators searched a residence on South Napoleon Avenue in Columbus on Friday. They seized 10 kilograms of fentanyl, or about 22 pounds, as well as a kilogram of methamphetamine and 10 ounces of heroin.

Fentanyl, a synthetic opioid, is several times more powerful than heroin. Two milligrams of the drug -- equivalent to a few grains of table salt -- is considered to be a deadly dose for more than 95 percent of the American public, according to the U.S. Drug Enforcement Administration.

Ohio authorities say fentanyl drove an increase in fatal overdoses in recent years. Hamilton County Health Commissioner Tim Ingram said local officials are also continuing to see changing forms of fentanyl requiring more of the opioid-reversal drug naloxone.

"You have to call 911 because you don't know what you've injected into the body," Ingram said.

The effect of the drugs depend on the size of the user, how potent it is and how long it can stay inside someone's body.

In Columbus, health officials plan to distribute 1,000 test strips to drug users to help them determine whether drugs are laced with the powerful opioid.