The video is not the one that has been widely distributed on YouTube, but has been reviewed by investigators, the sources said. It was recorded by a bystander using his iPad. It's not clear when the video surfaced but investigators learned of it late Monday.
The detective had originally been defended by officials as a deep undercover officer who failed to act in defense of the motorist because that might have unmasked him. But when the new videotape surfaced those officials felt duped, sources said.
The detective already had been criticized for waiting until Wednesday to come forward and acknowledge he was present at the scene.
This account of the new video was first published in the New York Post and confirmed by WNBC, the NBC owned-and-operated station in New York.
Police have so far charged three men in connection with the assault on the driver, Alexian Lien, who was pulled from his vehicle and beaten in front of his wife and daughter in an incident captured on videotape, and have a fourth in custody awaiting charges.
New York police released photos Tuesday of four more people who may have been involved in the brawl. They asked anyone with information to get in touch with New York Crime Stoppers at 1-800-577-TIPS (8477) or at the Crime Stoppers website.
Another man, Clint Caldwell, 32, was arrested Tuesday and charged with gang assault, assault and criminal mischief.
The investigation continues, a spokeswoman for the district attorney said.
"As we said from the beginning of the investigation, the NYPD and the District Attorney's Office are methodically scrutinizing the evidence to build the strongest possible cases in our continuing effort to hold accountable those responsible," Erin Duggan said.
Before Braszczok's name emerged, sources had described him as a veteran officer who has spent virtually all of his career undercover, sources say.
Michael Palladino, president of the NYPD detectives' union, the Detectives' Endowment Association, said on Monday that the undercover officer had to make a difficult decision.
"Compromising his identity could compromise all the work he's doing and his safety as well," said Palladino. "It's very difficult to lead a double life."
A law enforcement source told NBC News that some of Braszczok's duties in the past included undercover work within the Occupy Wall Street movement.
Braszczok participated in protests, attended meetings and lived among protesters during parts of the movement, providing information to the NYPD, the source said. It is unclear how long Braszczok did that work.
As many as five other off-duty police officers may have been riding with the hundreds of motorcyclists who flooded Manhattan on Sept. 29 as part of an annual biker event called Hollywood Stuntz.
More from NBC News Investigations:
- Undercover cop in SUV beating is vet assigned to intelligence
- Dangerous new breed of bike gang seeks Internet glory
- Feds: Huge drug-dealing website shut, owner linked to murder-for-hire plot