The U.S. Justice Department is suing Walmart over its role in the opioid crisis, Reuters reports.

The company ignored warning signs from its pharmacists and filled thousands of invalid prescriptions, the government said. The lawsuit alleges that Walmart unlawfully dispensed controlled substances from its pharmacies and unlawfully distributed the drugs to those pharmacies throughout the height of the prescription opioid crisis. This conduct resulted in hundreds of thousands of violations of the Controlled Substances Act, according to the lawsuit. The Justice Department is seeking civil penalties, which could total in the billions of dollars.

Walmart has 5,000 in-store pharmacies, the article notes.

“As one of the largest pharmacy chains and wholesale drug distributors in the country, Walmart had the responsibility and the means to help prevent the diversion of prescription opioids,” Jeffrey Bossert Clark, Acting Assistant Attorney General of the Civil Division, said in a news release. “Instead, for years, it did the opposite — filling thousands of invalid prescriptions at its pharmacies and failing to report suspicious orders of opioids and other drugs placed by those pharmacies. This unlawful conduct contributed to the epidemic of opioid abuse throughout the United States.”