The New York Police Department (NYPD) has launched its investigation into the killing of Eric Garner, but it could take up to four months to complete, the department’s head said.

NYPD Commissioner Bill Bratton said the decision last week by a grand jury not to indict officer Daniel Pantaleo for killing Garner meant that the internal investigation could kick off.

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“Now that the criminal investigation is concluded, the administrative investigation which focuses on violation of policies, procedures, rules and regulations can now move forward unimpeded,” Bratton said Sunday on CBS’s “Face the Nation.”

“Our investigation may take upwards of three to four months based on past experience, the number of officers, the number of witnesses.”

Bratton predicted that the investigation will conclude long before the Justice Department finishes its separate civil rights probe of the killing.

He promised a transparent investigation into possible violations of department policy, including a public trial if necessary.

Bratton will then have the final say on any potential punishments for Pantaleo, he said.

Pressed to comment on the widely circulated video of the chokehold killing, Bratton held back.

“I'm not free to comment at all on anything I observed or my feelings,” he said.

He acknowledged that the chokehold “looks awful,” but stressed that the grand jury found that it was not illegal.

Chokeholds are prohibited by NYPD policy, and the internal review will determine whether Pantaleo’s actions crossed the line.

In the meantime, the NYPD has a federal monitor during the civil rights investigation, is retraining its 22,000 officers on use of force and is rolling out smartphones to all cops, Bratton said. It is also undergoing a pilot program to equip some officers with body cameras.

In a radio interview on Sunday, Bratton praised the restraint shown by NYPD officers during the demonstrations following the Garner grand jury decision.

“I wish some of the protestors, professional agitators and anarchists would show the same restraint,” he added.

Bratton also he told radio host John Catsimatidis in an interview on New York's 970 AM that many of the demonstrators “are seeking to provoke the police.”

--This report was updated at 2:16 p.m.