Four men were arrested in Detroit over the past week for either threatening police or voicing support for the man who killed five police officers in Dallas last week.

One of the men allegedly made an explicit threat, writing, “Kill all white cops” on his Facebook page, while another wrote that this “needs to happen more often.” Yet another one of the men arrested praised the Dallas shooter as a “hero.” The Detroit police acknowledged that they’re on uncertain legal grounds after arresting the men, but they want to charge them anyway.

“I know this is a new issue, but I want these people charged with crimes,” Detroit police chief James Craig said. “I’ve directed my officers to prepare warrants for these four individuals, and we’ll see which venue is the best to pursue charges.”

However, experts cautioned that prosecuting people for things they’ve said could set a dangerous and tricky precedent.

“Certainly, posting that kind of thing on social media is a bad thought,” professor Larry Dubin of the University of Detroit Mercy School of Law told The Detroit News. “But having a bad thought isn’t necessarily a crime. There are a lot of issues at play here, and I think a lot would depend on what exactly was said.”

The outcome of these particular cases is far from determined, but they provide a valuable lesson to all Facebook users: watch what you say on the site. In particular, avoid saying anything that could be construed as threatening. After all, you never know who might end up seeing it.