U.S. Customs and Border Protection agents in Philadelphia last week discovered 110 pounds of fentanyl inside barrels of iron oxide being shipped from China, authorities said.

The seizure June 25 netted fentanyl with a street value of about $1.7 million, according to the agency. Stephen Sapp, an agency spokesman, said the cargo was flown into Chicago and then shipped by "truck in-bond" through Philadelphia, but officials did not specify where the seizure occurred or identify the cargo's intended final destination.

Fentanyl has quickly assumed an outsize role in Philadelphia's opioid crisis. This spring, the Medical Examiner's Office reported that the synthetic opioid was found in 846 overdose deaths in 2017 — double the total from 2016.

Last week, city officials said 20 crack cocaine users in West Philadelphia overdosed on fentanyl, two of whom died. Fentanyl is often mixed with heroin, cocaine, or other drugs to provide a cheaper and sometimes stronger high, officials said.

In last week's seizure, a narcotics detector dog smelled the drug inside barrels of iron oxide, leading officers to search and find 50 sealed bags of the white powdery substance, according to the customs agency. Further testing helped officers identify the substance as fentanyl, which customs officials said could be sold for $34,000 per kilogram on the street.

Nationally, nearly 1,000 pounds of fentanyl had been seized by customs officials in the first four months of 2018, the agency reported, up from 440 pounds in all of 2016.