It was not immediately clear what caused the helicopter to crash. Witnesses said it caught their attention because it was traveling faster and at a lower altitude than the helicopters that normally fly along the East River between Manhattan and Queens. The National Transportation Safety Board said Monday that 14 of its employees were en route to investigate the crash.

The helicopter, a Eurocopter AS350 owned by Liberty Helicopters, was flying as a private charter to take photos, Police Commissioner James P. O’Neill said at the news conference. A brochure for Liberty Helicopters, which is based in New Jersey, says it is the only company licensed to fly within 1,000 feet of the Statue of Liberty on all of its tours.

Its pilots have been “flying safely” for 28 years, according to the brochure. The company could not be reached for comment on Sunday night.

People who witnessed the crash said they saw emergency responders pull one person out of the water — a man who was wrapped in a white blanket — and place him in an ambulance at about 8 p.m. About an hour later, three people were taken out of the water and placed on stretchers. Emergency responders were seen performing CPR on them as they were placed into ambulances.