Federal authorities say they tracked more than a half-ton of chemicals that can be used to make the deadly opioid fentanyl to Texas and Tucson.

The effort helped lead to indictments against Chinese companies and employees revealed on Tuesday by the U.S. attorney general.

Homeland Security Investigations used phone numbers linked to chemical shipments to identify bitcoin wallets of Chinese companies infiltrated by undercover agents last year.

Authorities say most labs for producing fentanyl are likely in Mexico, and drug smugglers are very savvy to law enforcement.   

“For them it might make sense to see if they can get the precursor chemicals shipped into the United States to then export it outside of the United States to a location where fentanyl could be being produced,”  said Leo Lamas, HSI deputy special agent in charge in Tucson.

Merrick Garland said on Tuesday that the government knows who’s responsible for smuggling fentanyl ingredients into the U.S. as he announced the indictments.

Speaking next at a news conference, the Homeland Security secretary said Mexican drug cartels are at the root of the transnational crime.

“I can tell you that just based on seizure information this is the No. 1 entry point into the United States for fentanyl,” said Lamas

The main reason is Arizona’s geographic proximity north of the Mexican states Sinaloa and Jalisco and their namesake cartels.

EDITOR'S NOTE: This headline has been updated to correct that one Chinese company was involved in the shipment of chemicals to Tucson, according to U.S. officials.