A spokesman for the NYPD confirmed the deaths to The Associated Press early Monday.
The pilot freed himself from the wreckage and was later discharged from a hospital.
"One of the most hard parts of the operation we're told is the five people besides the pilot were all tightly harnessed so these harnesses had to be cut and removed in order to get these folks off of the helicopter which was upside down at the time and completely submerged", said FDNY Commissioner Daniel Nigro. "We don't have a great deal to report yet, but will provide regular updates". Investigators say a strap from a passenger's safety harness may have inadvertently wrapped around a fuel shut-off lever, resulting in engine failure which sent the aircraft plunging into the frigid water. The pilot quickly surfaced, however, clinging to a flotation device as a tugboat and then police boats approached.
"It was sinking really fast", witness Mary Lee, 66, told the New York Post.
News footage showed one victim being loaded into an ambulance while emergency workers gave him chest compressions.
"The reality of why they died wasn't because of impact or trauma", Richards said.
The Eurocopter AS350 went down in the waters between the boroughs of Manhattan and Queens just after 7 p.m. ET, according to a Federal Aviation Administration spokeswoman.More news: Paul Ryan and Other Republicans Try to Talk Trump Out of Tariffs
More news: Who will Charlotte Flair be facing at WrestleMania?
More news: Thailand girds itself for Trump trade war
A law enforcement official told The New York Times that Thompson worked for the helicopter tour company that operated the aircraft.
"There are a whole lot of questions about this crash", he said.
US Sen. Chuck Schumer of NY is expected to call for the FAA to suspend Liberty Helicopters' FAA operating certificate until their safety record and this crash are fully assessed, according to a press release from his office. He was vacationing in NY with his own young son when he heard about the crash, and told WFAA that his "heart sunk" when he heard Cadigan was on board. Three other passengers were injured.
Two years prior, another incident over the Hudson saw a Liberty Helicopter with eight people on board plunge into the river. He said he thought something was unusual when he saw the helicopter flying low before it crashed into the East River, so he started shooting video.
He said military pilots are usually trained in what's called a "helo dunker", in which pilots practice getting out of harnesses while submerged in water and flipped upside down.
New York Police Department Commissioner James O'Neill said there were "dozens of calls" to emergency services reporting the incident, which was also caught on video.
Witnesses described seeing at least one person emerge from the helicopter and wave his hands for help.