At least three people in the last year have been arrested for illegally selling guns using Snapchat, as detailed in a report from The Guardian. In two of the three cases, the men allegedly purchased guns in a state with lax gun laws, brought them into a state with restrictive laws, and used Snapchat to advertise their availability by posting pictures and videos of the guns. They were tracked down by agents from the Bureau of Alcohol, Tobacco, Firearms, and Explosives, and one has signed a plea agreement.

Snapchat bans ads that promote “firearms, weapons, ammunition, or related accessories,” and it says the app shouldn’t be used for “any illegal activities,” including buying and selling illegal weapons. A spokesperson for Snap said the company doesn’t allow illegal content to be promoted or distributed.

But those rules are a challenge to enforce: users can post whatever they’d like to their account, and it’s up to community members or automated tools to flag the issue. Since selling a gun can be done legally, it can be even harder to spot bad actors. In these cases, the sellers are said to have posted repeatedly about multiple weapons, attracting the attention of federal agents.

Other social networks have run into similar issues with illegal weapons sales. Earlier this week, The Wall Street Journal wrote about how gun sellers continue to skirt Facebook’s restrictions three years after it implemented a ban in response to illegal sales being negotiated on the platform. Instagram has also faced issues with sellers skirting its advertising ban. Sales made over these apps are often legal when they’re made by private sellers, as The Daily Beast noted in 2017, and that can make the enforcement of anything but a blanket ban — which Snapchat doesn’t have — a challenge.