Border officers in Arizona have seized more than 1.5 million fentanyl pills since last week, officials said Monday. 

The pills were discovered in eight loads beginning Thursday at the Nogales Port of Entry, Port Director Michael Humphries tweeted. 

They were found concealed in a variety of methods, including gas tanks, floors, doors and vehicle panels. 

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"Pills included blue, multi-colored, and rainbow colored," Humphries wrote. 

Border authorities have seen an uptick in fentanyl seizures in recent months. In October, U.S. Customs and Border Protection (CBP) officers nabbed more than half a million fentanyl pills in Nogales. 

The rise of the drug as the primary driver of the opioid crisis over the last two years has dominated news coverage in large part because of how dangerous it is, being potentially fatal in even small doses. According to the latest Centers for Disease Control and Prevention data, synthetic opioids such as fentanyl made up 82.3% of opioid-involved overdose deaths in 2020.

The drug is created primarily in Mexico using precursors from China, and it is then transferred across the land border of the U.S.

Fox News' Michael Lee contributed to this report.