(AP) -- Enough New York City police officers stopped writing tickets and making low-level arrests in the weeks since two officers were fatally shot that the number of summonses plummeted 90 percent, the city's police commissioner said Friday.

But William Bratton said the problem at the nation's police largest department is on the mend and the numbers are already going back up.

"I would describe it as a slowdown," Bratton said. "They never stopped working, 911 calls were responded to, arrests continued to be made, crime continued to go down."

Numbers for low-level arrests like fare beating in the subway and public drunkenness were down by half and summonses for criminal activity were down more than 90 percent after the Dec. 20 shooting deaths of Officers Rafael Ramos and Wenjian Liu in their patrol car.

The officers were targeted by a mentally unstable man who vowed online to kill two "pigs" in retaliation for the deaths of black men and boys at the hands of police around the country. The shootings caused a growing rancor between the rank and file and Mayor Bill de Blasio to boil over, as dozens of officers turned their backs on him at the hospital, and then thousands did at the funerals for the officers.

There had been chatter of a deliberate slowdown because of the startling change in police statistics but Bratton did not confirm it until Friday, after he had taken a comprehensive look. He said he didn't know the cause of the slowdown, but he has said he realizes morale among some officers is low.

Roy Richter, president of the police Captains Endowment Association, denied the drop in arrests was deliberate.

"Police officers never stopped their daily job of putting themselves in harm's way protecting New Yorkers," Richter said in a statement. "This past month we experienced an unparalleled drain on our resources policing protesters gone wild and horrible tragedy in the assassination of our two brothers in blue."

Bratton said he doesn't believe the slowdown affected safety and noted crime remains at historic lows. No officers were facing discipline.

"I don't think it's warranted," he said. "I'm very mindful of the extraordinarily stressful situations that the officers in the city found themselves in last month. And so we'll work to bring things back to normal."

One officer was suspended for insubordination during the period but it wasn't directly related to the slowdown.

Bratton said no police union gave any official instruction to slow police action. He met this week with the union leaders, and with borough chiefs to discuss the issue. "To encourage them to get out to the roll calls and start encouraging officers, now that we're out of the funerals and demonstrations, to get back to normal activity," he said.

The low enforcement activity prompted some to question Bratton's signature crime-fighting tool known as "broken windows," where low-level offenses were targeted to prevent more serious crime. But Bratton said Friday it had no major impact.

"The whole thesis of 'broken windows' is if over time you don't address an issue, over time it will create a larger issue," he said.