TAIPEI (Taiwan News) — U.S. federal agents have seized more than 11 million counterfeit 3M N95 masks in recent weeks as part of a fraud investigation related to the coronavirus pandemic.

The Department of Homeland Security (DHS) said the fake masks were made in China, according to The Wall Street Journal (WSJ). A million counterfeit N95 masks were confiscated on Wednesday (Feb. 17), bringing the total number of knockoff masks to more than 11 million, Homeland Security Secretary Alejandro N. Mayorkas said during a news conference, according to The New York Times.

After being tipped off by 3M, DHS officials said they have seized counterfeit items in five states — without specifying which ones — and have opened criminal investigations, but did not offer more details. 3M is the largest producer of N95 masks in the U.S.

Officials said grammatical errors or typos on packaging or user instructions are red flags for fake masks, The WSJ reported. Authorities said they have contacted around 6,000 hospitals, clinics, and others who may have accidentally purchased the fake Chinese masks, advising them to stop using them and contact DHS immediately.

“The seizure of these counterfeit surgical masks ensures the health and safety of our frontline health care workers, who so bravely combat the pandemic, by preventing them from receiving inferior personal protective equipment,” Mayorkas said.

The confiscation of fake masks is part of the DHS’ initiative to stop fraud and criminal activity related to COVID-19, nicknamed Operation Stolen Promise, according to The WSJ. Officials have seized more than 1,800 shipments of mislabeled, fraudulent, unauthorized, or prohibited coronavirus test kits, personal protective equipment, and other items.