Slain New York Officers 'Brought Back Horrible Memories,' Says Former NYPD Commissioner

Ray Kelly warns that "officers have to be on alert."

"Quite frankly the mayor ran an anti-police campaign last year when he ran for mayor so there’s a bit of a residue," said Kelly, who was the police commissioner when de Blasio was running for mayor.

While it has been three years since an NYPD officer was killed on the job, Kelly said police shootings were far more frequent decades ago.

"We have a history in this city, in the 70's, of these sorts of assassinations of teams of police officers and we saw more coming down the pike," said Kelly, an ABC News consultant. "Actually, in 1972, there were 12 police officers killed."

Kelly said that "we don't know" if Saturday's shooting will lead to copycat attacks.

While the NYPD announced that at least two police vehicles will respond to every call as a security precaution, Kelly suspects that will not stay in place for long.

"I think cooler heads will prevail. As I said, there was a lot of emotion last night," Kelly said. "You may see a little of that early on here ... but I think that will cool down pretty quickly."

The former police commissioner also pushed back against the presumption that police may recoil after the shooting.

"I've never seen officers back off from their sworn duty," he said.