The official said federal authorities will investigate the July death of Eric Garner, who was confronted by the officer on suspicion of selling loose, untaxed cigarettes.
Earlier Wednesday, a New York grand jury chose not to indict Officer Daniel Pantaleo. Staten Island District Attorney Daniel Donovan said jurors found "no reasonable cause" to bring charges.
The official spoke on condition of anonymity since the department had not yet announced the investigation.
Federal authorities conducting a similar investigation in the Ferguson, Missouri, police shooting case.
Jonathon Moore, an attorney for the victim's family, says he is "astonished by the decision."
Garner was stopped on suspicion of selling loose cigarettes in July, and amateur video shows the black man telling the officers to leave him alone before Pantaleo, who is white, used what appeared to be a banned chokehold.
The medical examiner found that the chokehold contributed to the death of 43-year-old Eric Garner.
Attorney Stuart London said Pantaleo testified last month for a couple of hours.
"My client was gratified that they took the time to listen to everything he said and he knows his future is in their hands," London said.
Pantaleo's partner, Justin D'Amico, testified after being granted immunity from prosecution, he added.
Staten Island District Attorney Daniel Donovan told reporters Tuesday that jurors diligently listened to the evidence.
A small group of protesters marched in the cold outside U.S. Attorney's Office in Lower Manhattan Tuesday, the remnants of the Ferguson protests, but few doubt the larger demonstrations from last week will be back soon.
"When the decision comes down, regardless of what the decision is, there will be some demonstrations," Donovan said.
At a news conference Wednesday, Mayor Bill de Blasio urged any demonstrators to protest peacefully.
"I believe in the right to peaceful protest as part of a democracy, and anyone who has views as a result of whatever the decision is, if they want to exercise peaceful protest, we welcome that and respect that," he said. "But we also will keep order."
De Blasio also urged protesters to heed the message of Eric Garner's son, Eric Snipes.
"He said that anyone who wants to express themselves to do so peacefully," de Blasio said. "And for his own son, who lost his father, said honor my father by doing any protest in a peaceful manner, I thought it was very powerful. He was not just talking about how you go about making changes and reforms, he was talking about the nature of his own father as a peaceful man, and honoring his memory by being just as peaceful as that man was."