Owlet has stopped selling its lineup of smart baby monitoring socks, which are supposed to track a baby’s vital signs and sleep patterns, after receiving a warning letter from the Food and Drug Administration (FDA). First reported by Deseret News, the FDA’s letter states that Owlet’s Smart Socks are considered medical devices, as they provide heart rate and oxygen level notifications, and that the company has been selling them without the proper “marketing approval, clearance, or authorization” from the FDA.

Owlet has since pulled its family of Smart Socks, as well as any bundles that include the device from its site. “The Owlet Sock family of products is currently unavailable,” the Smart Socks’ product page reads. “Check back in the coming weeks to see the newest addition to the nest.” Owlet Smart Socks still appear to be available on other online marketplaces, like Bed Bath & Beyond, Amazon, and BuyBuyBaby — for the time being at least. Owlet Smart Socks will still be available for purchase outside of the US.

The FDA explains that the Smart Socks are medical devices, specifically because they measure blood oxygen saturation and heart rate with the intent to “identify (diagnose) desaturation and bradycardia and provide an alarm to notify users that measurements are outside preset values.” As mentioned in the letter, Owlet previously asserted that its Smart Socks are low-risk products, not medical devices. The FDA has apparently been notifying the brand that this isn’t the case since 2016.

Owlet posted a response to the FDA on its site, noting that it plans on complying with the FDA’s request, and will seek marketing approval for its features that track heart rate and blood oxygen levels. The company also states that the FDA didn’t “identify any safety concerns about the Smart Sock,” and reassures existing sock owners that “there has not been any change” to its functionality.

Owlet also hints at introducing a new, similar product in its letter, stating that it plans on offering “a new sleep monitoring solution,” which will be available “soon.” The Verge reached out to Owlet about its upcoming product and whether that will replace the Smart Sock, but didn’t immediately hear back.

Smart baby monitors, like Owlet’s Smart Socks, have been criticized in the past. Doctors have suggested that they may actually put infants at risk, due to the fact that they’re not classified as medical devices, and aren’t subject to certain regulations and oversight.

“The Smart Sock’s safety has been validated by third parties, in which it was shown to be safe,” Owlet asserts in its post. “In addition, the letter we received from the agency [the FDA] did not identify any safety concern about the Smart Sock.”