In the balcony of Baltimore’s Hippodrome Theatre last week, during intermission of a performance of “Fiddler on the Roof,” a man began shouting “Heil Hitler, heil Trump.”

Audience member Rich Scherr said, “People started running, I’ll be honest, I was waiting to hear a gunshot. I thought, ‘Here we go.’ ” Mr. Scherr feared a repeat of the recent slaughter at Pittsburgh’s Tree of Life Synagogue. His fears were reasonable, given the FBI’s recent arrest of a man who said on social media that the Pittsburgh shootings were “a dry run” and that “there was more to come.”

Yet the Baltimore police — claiming to be straitjacketed by the First Amendment — did nothing except to issue “a stop ticket” to the big mouth as they escorted him from the theater. No arrest. No warning to cease and desist. Just a tap on the wrist.

And yet we are in America, where 99 years ago, U.S. Supreme Court Justice Oliver Wendell Holmes’ ruled in Schenck v. U.S. that you cannot shout “fire” in a crowded theater. Actually, Holmes’ precise words were: “The most stringent protection of free speech would not protect a man falsely shouting fire in a theatre and causing a panic.” The Supreme Court, in its 1969 Brandenburg decision limited Schenck by ruling that speech was protected unless it was likely to incite imminent lawlessness like a riot.

We believe that the Baltimore police should have acted, when they failed to arrest the Baltimore provocateur. Baltimore’s political leaders should instruct the police chief to open an investigation of the perpetrator whose behavior wasn’t only designed to hurt feelings but to cause panic in an enclosed public venue.

While they are at it, there should be a full review of his public social media postings to see if he is a threat of imminent anti-Semitic violence. Perhaps his Hippodrome Theatre outrage was just a trial run, analogous to John Wilkes Booth’s casing Washington’s National Theater before actually shooting President Lincoln and then jumping onto the stage exultantly shouting: “Sic Temper Tyrannis!”

We live in a time in America when violent bigots have brutally breached the sanctum of churches and synagogues to murder innocents for the sin of their faith and skin color. Beyond toothless photo-ops, it’s past due for our political leaders and law enforcement to wake up to the new reality and make appropriate changes to policy to safeguard all citizens. This extends to our nation’s campuses, where bullies have shut down free speech, where pro-terrorist groups are permitted to convene national conventions at places like UCLA, where Antifa “activists” on and off campus merrily put a torch to our civil discourse and protections.

What happened in Baltimore is not merely a discomfiting incident in a theater. In our social-media-driven age, hate-filled incitements left undealt with will inspire lone-wolf wannabees to go the next step. This isn’t an exaggeration — but rather America’s worst nightmare possibly coming to a community near you.

Rabbi Abraham Cooper is associate dean of the Simon Wiesenthal Center and Global Social Action director. Harold Brackman is long-time consultant for the Simon Wiesenthal Center and its Museum of Tolerance and co-author of “From Abraham to Obama: A History of Africans, African Americans, and Jews” (Africa World Press, 2015). They may be reached at information@wiesenthal.com.