New York District Attorney Cyrus Vance Jr. opened an investigation into the Saturday episode, which was captured on video and widely disseminated on the Internet, the source told The New York Times.
A Vance spokeswoman told the newspaper, "The district attorney's office takes all allegations of police misconduct seriously."
She said the approximately 80 arrests made at the protest were "being reviewed under the standard procedure."
Vance's probe follows New York Police Commissioner Raymond Kelly's announcement Wednesday the department's Internal Affairs Bureau and the city's Civilian Complaint Review Board would look into Deputy Inspector Anthony Bologna's pepper spray use on female demonstrators. The review board is an all-civilian board that investigates civil complaints about alleged New York police misconduct.
Bologna was identified Wednesday in another video spraying pepper spray at other demonstrators. That video, evidently from a moment after the first pepper spray shooting, showed photographer Andrew Hinderaker, 23, with a press card around his neck, being hit with pepper spray.
Bologna will "cooperate with whatever investigative body the police commissioner designates to perform this review," Deputy Inspector Roy Richter, president of the union representing upper-level police officers, was quoted in the Times as saying.
Richter said Bologna used the pepper spray because he was concerned "for the safety of officers under his command and the safety of the public."
"The limited use of pepper spray effectively restored order without any escalation of force or serious injury to either demonstrator or police officer," he said.
The department's Patrol Guide says pepper spray should primarily be used if a suspect is resisting arrest, or for protection. But it does allow it to be used in "disorder control" by officers with special training.
The police department did not say whether Bologna had special training.