A federal investigation in San Diego has led to drug-related charges against three people in what authorities said was one of the largest seizures of Fentanyl in the U.S (file photo)

Federal authorities in San Diego say they have seized one of the largest loads ever in the U.S. of the potent synthetic opiate Fentanyl.

An indictment unsealed Monday says federal drug agents seized over 97 pounds of the drug, most of it from a house in San Diego.

A U.S. attorney's statement says the drug is so powerful that the amount represents millions of lethal doses.

Drug traffickers use it to strengthen heroin or make counterfeit painkiller pills.

Three people, Jonathan Ibarra, 45, Hector Fernando Garcia, 46, and Anna Baker, 30, were indicted for possession of the drug with intent to distribute.

Just 3 milligrams is enough to kill an adult male - so the almost 100 pound seizure represents more than 14 million lethal doses (file photo)

Court papers say the defendants discussed having Baker smuggle the drugs on three separate trips.

In November, agents stopped Baker's rented vehicle and found about 33 pounds of Fentanyl, then discovered 66 more pounds of the drug during a search of her house.

On the streets, Fentanyl is sold alone as powder, added to heroin or made into counterfeit painkiller pills.

The drug has been overrunning cities throughout America largely due to trafficking by Mexican cartels.

Authorities have said Mexican drug cartels produce the drug in labs using precursor chemicals from China.