The FDA has cracked down on more companies trying to take advantage of the COVID-19 pandemic, issuing warning letters to four firms that sold unapproved COVID-19 tests online for sample collection in patients’ homes.

The agency issued the warning to CoreMedica, Fair Price Labs, Holistic Health and AkivaMed for their at-home blood or stool sample tests. All the tests required users to send their samples to the companies for testing.

The FDA said that it has not cleared any at-home stool or blood test kits for COVID-19 and ordered the companies to stop selling the unapproved products.