The pilot, 33-year-old Richard "Rick" Vance, was the only one to survive the crash. He made a mayday call as the helicopter foundered and reported the engine had failed.
The helicopter crashed near 89th Street in Manhattan around 7 p.m. after taking off from Kearny, New Jersey.
It was removed from the water Monday and taken by barge to the Brooklyn Marine Terminal, where the wreckage will be dismantled and examined.
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Vance of Danbury, Connecticut, was able to free himself, but the passengers were not able to get out. Vance was taken to New York Presbyterian Hospital, where he was treated and released.
He told police he noticed signs of engine trouble and thought about landing in Central Park but ruled it out and headed toward the river.
It took FDNY and NYPD divers awhile to free the passengers from the helicopter, which was upside-down. The passengers were tightly harnessed, so responders had to work to get them freed.
RELATED: Harnesses likely kept passengers from escaping crash, experts say
Two men were pronounced dead at the scene, while two other men and a woman were pronounced dead at Bellevue Hospital and New York Presbyterian Hospital. Two of the victims are from Manhattan, two from Dallas, and one is from Argentina.
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The aircraft was operated by Fly Liberty Charter for a private photoshoot where passengers take photos with the doors of the helicopter open. It took off from HHI Heliport in Kearny, NJ.
National Transportation Safety Board member Bella Dinh-Zarr said investigators would look for cameras and electronic devices on board that may have captured the flight's final moments. She said the agency hasn't spoken yet to the pilot but hopes to do so.
"Our mission will be to understand not just what happened but why it happened and prevent it from happening again," said Dinh-Zarr.
Witnesses on a nearby waterfront esplanade said the helicopter was flying noisily, then suddenly dropped and quickly submerged. But the pilot appeared on the surface, holding onto a flotation device as the tugboat and then police boats approached.
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"We were having dinner and we noticed a red helicopter going full speed towards the water. It almost looked very surreal and next thing we know it's approaching the water slowly and then it just completely crashed and then sunk. Immediate reaction was to call 911," said eyewitness Arineh Nazarian.
LiveATC.net released the audio from Air Traffic Control.
Pilot: "Zero lima hotel, mayday mayday."
Tower: "Lima hotel you ok?
Pilot: "(???) engine failure"
Tower: "I'm sorry, say again?"
Pilot: "(???) engine failure"
Tower: "You're coming up broken, say that one more time?"
Unknown pilot: "He had an engine failure over the East River, Lima hotel."
Tower: "Ok are you requiring assistance?"
Unknown pilot: "It was a mayday call, LaGuardia."
Tower: "Ok, got it"
Unknown: "LaGuardia did you have his position, at the, the last position reported for him?
Tower: "Yeah I do, thank you."
The fuel cut off is located on the floor and the harnesses that the passengers were wearing connect near the floor.
The pilot apparently said that when the passengers are moving around while they are getting pictures, their harnesses got tangled on the floor.
He told investigators that a tether from a front passenger's harness wrapped around the fuel shut off level, which had accidentally cut off the fuel supply to the engine, resulting in the engine failure. At this point, the engine could not be restarted and the aircraft plunged.
The pilot attempted to inflate the pontoons on the helicopter's skids prior to hitting the water. One of the pontoons did not inflate which is why the helicopter listed to one side upon impact and rolled over in the river, filling the open doors with water.
The passengers wore heavy harnesses, attached to the floor of the helicopter, that are more difficult than normal harnesses to unattach. The doors were open for the photo shoot, allowing water to rush in and quickly sink the chopper.
The chopper was equipped with inflatable rafts, designed to upright in an emergency water landing. But somehow they failed.
"The pilot did a really good job auto rotating onto the water, landing upright. If the floats had deployed properly, and the aircraft hadn't had rolled over, this would not have been the tragedy it was," said ABC News contributor Stephen Ganyard.
The NTSB has a team of investigators on the scene investigating the crash and examining the helicopter.
NTSB Go Team gathers information on scene in New York City while awaiting salvage of the helicopter that crashed in the East River, New York, March 11, 2018 pic.twitter.com/ySmnPhoATC— NTSB_Newsroom (@NTSB_Newsroom) March 12, 2018
The helicopter is now in police possession.
The aircraft was owned by Liberty Helicopters, a company that offers both private charters and sightseeing tours popular with tourists.
The company referred inquiries to authorities, saying it was focused on the victims' families and the investigation.
The skies over New York constantly buzz with helicopters carrying tourists, businesspeople, traffic reporters, medical teams and others.
In 2009, a Liberty Helicopters sightseeing craft of the same model as the one in Sunday's wreck collided with a small, private plane over the Hudson River, killing nine people, including a group of Italian tourists.
A helicopter crash in October 2011 in the East River killed a British woman visiting the city for her 40th birthday. Two other passengers died weeks later as a result of their injuries.
A helicopter on a sightseeing tour of Manhattan crashed into the Hudson River in July 2007, shaking up the eight people aboard but injuring no one. In June 2005, two helicopters crashed into the East River in the same week, injuring a total of 15 people.
(The Associated Press contributed to this report.)
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