PHOENIX (AP) — U.S. agents in southern Arizona said Thursday they seized up to 440 pounds (about 200 kilograms) of what they suspect is a precursor chemical often used to manufacture the dangerous drug fentanyl, a chilling sign that producers may be moving to manufacture the deadly synthetic opioid on American soil.

The powdered chemical was being transported by agents Thursday morning from a residence and warehouse in Tucson, where it had been sent in recent months in a series of suspicious packages from China that did not identify their contents, said Leo Lamas, the deputy special agent in charge for Homeland Security Investigations in Tucson.

There were no immediate reports of arrests after agents with federal search warrants entered the locations and found no people inside.

“If criminal entrepreneurs start producing fentanyl here, it could become a frightening situation for the United States,” said Mike Vigil, former head of international operations for the Drug Enforcement Administration. “If the chemicals do turn out to be precursors, this could really aggravate fentanyl distribution and overdoses.”

“Once you have a drug trend like this, it can catch on and spread quickly,” Vigil added.

Lamas said the agency launched the investigation several months ago after learning suspicious packages containing the chemical, which he did not name, were being shipped to the two locations from China. DEA agents and Tucson police provided important support, he said.

He said the agency was still investigating who was involved in shipping and storing the chemicals and did not yet know if they were part of a local crime group or an international drug organization.

If further investigation shows that the chemicals were destined for fentanyl production, Thursday’s seizure could mark a new model for how it is manufactured, making it more readily available to consumers in the United States.

Already, the DEA calls fentanyl the deadliest drug in the nation. Two-thirds of the 107,000 overdose deaths in the country in 2021 were attributed to synthetic opioids like fentanyl, the U.S. Centers for Disease Control and Prevention says.

Originally developed as a legal drug to treat intense pain from ailments including cancer, Mexican drug cartels in recent years have produced most of the illegal fentanyl seen in the U.S., smuggling it inside vehicles or strapped to pedestrians crossing at ports of entry along the international border.

“I consider fentanyl to probably be the most dangerous controlled substance we’ve ever had to deal with,” said Lamas.

The drug is sold in various forms, including blue pills designed to look like oxycodone, known locally in southern Arizona as “Mexican oxy.” In its powder form, fentanyl is often mixed with other drugs like heroin or methamphetamine.

But investigators in recent months have seen incipient signs that producers may be trying to make fentanyl inside the U.S., said Lamas.

“Over the last, I would say a year or so, we have started to uncover pill presses domestically in the United States,” he said.

“Certainly, these criminal organizations know that the emphasis is at the border,” Lamas said of interdiction efforts. Importing precursors from China and producing fentanyl domestically would be “just another mechanism that these criminal organizations will attempt to get their products sold in the United States,” he added.

Cheap to produce and buy, fentanyl has exploded in popularity in recent years. But even a tiny dose can be fatal because it’s 50 times more potent than heroin and producers don’t tend to worry about quality control when pressing the powder into pills or lacing it with other drugs.

Addiction to fentanyl has spread through homeless populations in Los Angeles and other large western cities where it is sold in small doses at low cost.

“What makes it so much more dangerous is because it really doesn’t discriminate in terms of economics,” said Lamas. “It’s available to anybody.”