Story highlights Source: An off-duty police officer was among motorcyclists who saw incident

He didn't report what he saw to superiors until 4 days later, the source adds

The officer, who works undercover, was riding with his motorcycle club

Bikers chased the SUV, then dragged out driver and beat him, police say

A New York police officer saw much of a confrontation between fellow motorcyclists and a Range Rover driver -- which ended with five injured, including the SUV driver -- but didn't step in, a law enforcement source said.

The police officer, who works undercover, was off-duty and riding with the motorcycle club he belongs to when the incident occurred Sunday. But he didn't tell his superiors about what happened until Wednesday, the source said.

It's not clear why the officer waited so long to report what he saw. The officer, who has hired a lawyer, is being investigated by the New York Police Department's internal affairs unit. But he may not have been legally obligated to immediately intervene, according to the same source.

Asked about this officer, Deputy Police Commissioner John McCarthy declined comment, saying only, "The investigation is ongoing."

The Manhattan district attorney's office, which is pursuing the case, also didn't comment. However, it appears likely that prosecutors, at a minimum, will want to interview the officer as a witness.

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The off-duty officer being in the middle of the mix is the latest twist in a saga that's raised questions, and stirred debate, around New York and beyond.

Police say the incident -- some of which was captured on video -- began Sunday afternoon when a man driving his wife and their 2-year-old daughter in a Range Rover on Manhattan's West Side Highway struck a motorcycle, which had slowed in front of his SUV. This cyclist was slightly injured.

When Alexian Lien, the SUV driver, pulled to a stop, other bikers surrounded the vehicle, hit it and spiked its tires, police said.

Lien then pulled away, plowing into three more bikers, including Edwin Mieses, who was critically injured.

On the video, which was shot from one of the riders' helmet cameras, it's around then, as the Range Rover stops further north on the road that runs all along the western side of Manhattan, that a man got off his motorcycle, police spokesman Sgt. Carlos Nieves said Friday.

This helmeted man -- who Nieves identified Friday as 35-year-old Robert Sims -- then walks quickly over to the SUV and opens the driver's door. Lien then drives off north, with his door slightly open, according to Nieves.

After a chase, the motorcyclists corner the SUV. The video shows one biker using his helmet to smash the driver's side window.

Police said the bikers then dragged Lien from the vehicle and beat him. His wife and daughter were unharmed.

Several questioned, one arrested and charged

Sims was questioned Friday by police, presumably about why he allegedly walk up the Range Rover and opened one of its doors.

He drove to a Brooklyn police station voluntarily and is not under arrest, Nieves said.

Authorities said late Thursday they'd identified, and will soon apprehend, what appears to be a different motorcyclist who played a role in Lien's beating, a law enforcement official with knowledge of the investigation told CNN.

This biker is suspected of bashing in the driver's window, according to the official.

Police previously arrested Christopher Cruz, the biker seen slowing in front of the Range Rover before being bumped.

Cruz, 28, was in court Wednesday on misdemeanor charges including reckless driving. He was released after posting $1,500 cash bail and a $15,000 insurance bond. In addition, his license was suspended, and he was ordered to surrender his passport.

Cruz is not guilty, according to his lawyer.

"His motorcycle was struck, and he stood right there," his attorney H. Benjamin Perez said. "He never assaulted this man. He never tried to assault him in any way. And he does not know any of the other motorcyclists who were involved in this beating."

A second biker was released Wednesday when authorities determined that he may have been trying to help.

Mieses' family says he is the real victim. "All of his ribs are fractured. His lungs are so badly bruised that he's still on a ventilator," Yolanda Santiago, his mother, told CNN affiliate WCBS.

Mieses' wife, Dayana, told CNN affiliate WBZ that he got off his bike to help the SUV driver. She blamed Lien. "He got scared, he peeled off, and he paralyzed my husband on the way," she said.

The motorcyclist who shot helmet-cam video of the incident has been questioned by police, who seized the raw video of the incident from his home in Bellport, on Long Island. An edited version of the video, which authorities say ended moments before Lien was pulled from his vehicle, had been posted on the Internet.

SUV driver acted 'to protect the lives' of his family

Lien was treated at a hospital for slashes to his face.

The family was on an outing to celebrate the couple's wedding anniversary, Lien's wife said Thursday in a written statement.

"My husband was forced under the circumstances to take the actions that he did in order to protect the lives of our entire family," she said.

"We know in our hearts that we could not have done anything differently, and we believe that anyone faced with this sort of grave danger would have taken the same course of action in order to protect their family."

It was Lien's wife who made the last of three 911 calls the family placed during the incident, CNN has learned.

New video emerged Thursday showing bikers riding on sidewalks before Sunday's rally. It prompted state Sen. Adriano Espaillat, D-New York City, to call for stricter enforcement of traffic laws.

"I don't see why you can have 500 motorcycles that are doing wheelies and that are out of control stopping traffic, totally stopping traffic in the highway or in some streets and doing whatever they want," he told reporters about the video, which his office released.

Bikers have called for Lien to be charged.

"That wasn't fear, that was aggression -- he ran over three bikes," a man who identified himself as Jose told reporters Thursday night. "Are we saying, if you feel nervous you can kill somebody? You can paralyze somebody? I think we need to charge him immediately."

Another biker called for cool heads to prevail. "We are not here to blame anyone, we are not here to point any fingers," Albert Elkerson said. "The true question is how could we have avoided what happened last Sunday, and what can we do to prevent that."