One tweet that led to an arrest said that Micah Johnson, the man who shot police officers in Dallas last week, was a hero. None of the men have been named, nor have they been charged.

"I know this is a new issue, but I want these people charged with crimes," said Detroit Police Chief James Craig. "I've directed my officers to prepare warrants for these four individuals, and we'll see which venue is the best to pursue charges," he added.

Five police officers were killed in the Dallas shootings, constituting the highest number of police casualties in an attack since September 11. And as a result, law enforcement officials everywhere are suddenly much more sensitive to threats against their lives.

But one result has been that several police departments across the country have arrested individuals for posts on social media accounts, often from citizen tips — raising concerns among free speech advocates.