Last year, responding to a series of civilian killings involving the police, Gov. Andrew M. Cuomo of New York signed an executive order granting Mr. Schneiderman the power to investigate and prosecute police-related deaths.

Local district attorneys had come under fire in several of the killings, particularly in the choking death of Eric Garner on Staten Island, for their potentially conflicted roles in prosecuting police officers, with whom they work closely and regularly. In announcing the order in July 2015, Mr. Cuomo, a Democrat, said that New York would become the first state in the country to “create an independent prosecutor who does not have that kind of connection with the organized police departments.”

The governor’s order drew criticism from many of the state’s district attorneys — including Ken Thompson in Brooklyn — who argued that despite their tight relationships with police officers, they, the duly elected top law-enforcement officers of their counties, and not statewide prosecutors, were best suited to handle such cases. After suing Rensselaer County’s district attorney, Mr. Schneiderman and his special investigations and prosecutions unit wrested control of the case involving the fatal shooting of Edson Thevenin, who was killed by the police after a chase in the upstate city of Troy in April.

Shortly before the news about the charges was reported on Monday afternoon by The New York Post, Mr. Small’s girlfriend, Zaquanna Albert, wrote a post on Facebook, saying, “God got us.”

Reached by telephone, Ms. Albert declined to speak about the charges against Officer Isaacs. A woman who later answered her phone said, “She’s stressed out,” and hung up.

Roger Wareham, a lawyer representing Ms. Albert, said he welcomed the charges against Officer Isaacs as “a very positive step forward,” adding that his client was “satisfied.”

“I think the charges are proper, that they reflect the gravity of the crime that he committed,” Mr. Wareham said. “But I will withhold judgment because the history has been really bad in terms in terms of state prosecution of police officers.”

“The signal that that sends to the police,” he continued, “is that even when you are caught cold-blood, cold-handed guilty, that you are still going to get a pass because you are a police officer. I think it’s that message to police officers that results in the killings that we see across the country.”