(Reuters) - A lecturer at a New York City college of criminal justice who wrote on social media that he teaches “future dead cops” was placed on leave on Friday, the school’s president said, as the city’s mayor called the academic’s comment “vile.”

John Jay College of Criminal Justice adjunct professor Michael Isaacson has taught courses in economics and is a self-described member of the anti-fascist, or antifa, movement.

Many students at John Jay eventually join the New York Police Department.

On Friday, Pat Lynch the head of New York’s largest police union, the Patrolmen’s Benevolent Association, sent John Jay’s president a letter calling for Isaacson’s dismissal. Lynch highlighted an Aug. 23 tweet.

“Some of y’all might think it sucks being an anti-fascist teaching at John Jay College but I think it’s a privilege to teach future dead cops,” Isaacson wrote in the post on Twitter, according to a screen-grab from the union.

Lynch’s letter accused Isaacson of promoting violence against police.

The criticism of Isaacson’s social media post follows heightened scrutiny of law enforcement over officers’ use of force against minorities. The country has also seen a number of targeted killings of police officers, including the shooting death of five officers in Dallas last year.

John Jay College President Karol Mason said in a statement on Friday that John Jay faculty members had received death threats and that Isaacson was placed on administrative leave “out of concern for the safety” of students, faculty and staff. The college is reviewing the matter, Mason said.

“I am appalled that anyone associated with John Jay, with our proud history of supporting law enforcement authorities, would suggest that violence against police is ever acceptable,” Mason said.

New York Mayor Bill de Blasio on Friday wrote on Twitter the city “won’t stand for the vile anti-police rhetoric of Michael Isaacson and neither should John Jay College.”

Isaacson appeared on Fox News on Thursday, where during an interview by host Tucker Carlson he defended the antifa movement.

Isaacson’s television appearance appears to have drawn increased attention to his Aug. 23 Twitter post.

Isaacson, in a statement on Friday, said he critiques “policing as an institution which operates at the behest of a state that increasingly represents the weapons and prison industry.”

“My biggest regret is putting my students and the John Jay faculty and staff at risk,” he said in a separate statement by email.