Mexican soldiers seized more than a half million fentanyl pills in a raid on what the army’s announcement Wednesday called the largest synthetic drug lab found to date.

The army said the outdoor lab was discovered in Culiacan, the capital of the northern state of Sinaloa. Sinaloa is home to the drug cartel of the same name.

Soldiers raided the lab Tuesday and found almost 630,000 pills that appear to contain the synthetic opioid fentanyl. They also reported seizing 282 pounds (128 kilograms) of powdered fentanyl and about 220 pounds (100 kilograms) of suspected methamphetamines.

“This is the highest-capacity synthetic drug production lab on record during this administration,” the army said in a statement.

Mexican drug cartels produce the opioid from precursor chemicals shipped from China, and then press it into pills counterfeited to look like Xanax, Percocet or Oxycodone. People often take the pills without knowing they contain fentanyl and can suffer deadly overdoses.

The bust came on the same day that the U.S. Senate Foreign Relations Committee held a hearing on the the huge number of U.S. fentanyl overdoses that occur annually, currently around 70,000.

The committee’s chair, Sen. Bob Menendez , a Democrat from New Jersey, called on Mexico to do more.

“This means asking Mexico to do more to disrupt the criminal organizations from producing and trafficking fentanyl, although a politicized judiciary and incidents of Mexican security forces colluding with drug cartels will make that difficult,” he said.

(AP)