As the battle against the opioid epidemic rages on, the Food Drug and Administration (FDA) and the Drug Enforcement Administration (DEA) are focusing on a new target—the internet, where shady websites continue to illegally market and sell opioids.

This week, the two agencies sent warnings to four online networks that operate a total of 10 websites that illegally sell unapproved and misbranded versions of several dangerous, addictive medications, including tramadol. The websites targeted include Divyata, Euphoria, JCM Dropship, Healthcare Pvt Ltd. and Meds4U.

In the letters, authorities call for each company to immediately cease selling opioids to consumers in the United States. They also demand responses from each of the companies within 15 business days and request information on what specific actions they will take to address the violations noted. The letters also warn of potential legal action should they fail to cooperate.

“We cannot allow rogue online pharmacies to continue to fuel the crisis,” said acting FDA Commissioner Ned Sharpless, MD, in a press release. “We remain committed to using all available regulatory and enforcement rules to stop the illicit flow of opioids online.”

The DEA also notes that the networks behind these websites violated the Controlled Substances Act by failing to register their online pharmacies with the agency, despite knowingly advertising the sale of controlled substances.

These letters follow similar warnings sent to and in April. Warning letters were also sent to dozens of websites last year on two different occasions. Even Facebook’s Mark Zuckerberg faced criticism from Congress last year for not doing enough to prevent the illegal sale of opioids on the social media platform he founded.

Every day, more than 130 people in the United States die of an opioid overdose, according to the National Institute on Drug Abuse.