The organization will be gathering information about the pilot, the environment and Monday's weather and releasing a preliminary report in about two weeks, NTSB Air Safety Investigator Doug Brazy said at a press conference Tuesday.
Officials stress that the investigation is in its very early stages, but weather may have played a large role, law enforcement officials briefed on the probe told ABC News. They said that the pilot radioed back to the heliport soon after taking off to say that he was in trouble because of the weather and low visibility.
To try to get to safety, he ascended higher than helicopters usually fly in hopes of getting above the clouds, but he could not. He became disoriented, and though his original plan was to fly south, he wound up flying west into a no-fly zone.
Then the helicopter crash-landed.
The pilot, who has been identified as Tim McCormack, was an experienced pilot who likely did all he could to avoid putting others' lives at risk, family said.
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The crash happened Monday afternoon after an erratic trip across some of the nation's most restricted airspace. It briefly triggered memories of 9/11, but authorities were quick to state that they did not suspect terrorism and believed it to be an accident.
McCormack had taken off from 34th Street Helipad in a privately owned helicopter around 1:32 p.m. en route to Linden, New Jersey.
Around 1:45 p.m. ET the aircraft crash-landed on the roof of 787 7th Avenue, according to officials. That address is between 51st street and 52nd street, which is in the vicinity of both Rockefeller Center and Times Square. The building, called the AXA Equitable, is about 750 feet tall, the Associated Press reports.
The crash, which shook the building, sparked a fire and forced office workers to flee on elevators and down stairs, witnesses and officials said.
A woman who wished not to be identified told WABC-TV, our sister station in New York City, that she works on the 30th floor of the building. She said the impact was "not like anything I've ever felt before."
McCormack was the only person aboard, and there were no other reports of injuries, authorities said.
It was not immediately clear what caused the crash, or why the Agusta A109E was flying in a driving downpour with low cloud cover and in the tightly controlled airspace of midtown Manhattan. A flight restriction in effect since President Donald Trump took office bans aircraft from flying below 3,000 feet (914 meters) within a 1-mile (1.6 km) radius of Trump Tower, which is less than a half-mile (0.8 kilometers) from the crash site.
The city currently allows helicopters to take off and land from three heliports, one each on the East and West sides and in downtown Manhattan. All of the facilities border rivers.
It was once more common for helicopters to take off from private Manhattan rooftops, the most famous of which was on the tower then known as the Pan Am building. In 1977, four people waiting on the roof were killed when a helicopter toppled over and a rotor blade broke off and hit them. A fifth person, a pedestrian, was killed by falling debris.
That spurred a push to close down private helipads.
Still, the city has seen a string of helicopter accidents since. The most recent was just last month, when a chopper crash landed in the Hudson River near a busy Manhattan heliport. The pilot escaped mostly unscathed.
Five people died when a sightseeing helicopter crashed into the East River last year. Three people died in another crash into the same river in 2011. Back in 2009, a sightseeing helicopter collided with a small plane and killed nine people not far from the scene of Monday's mishap.
In 2006, New York Yankees pitcher Corey Lidle's single-engine plane slammed into the 20th floor of a building on Manhattan's Upper East Side, killing Lidle and his flight instructor. It was not clear which one was piloting the plane.
The National Transportation Safety Board concluded the pilot misjudged a narrow U-turn before veering into the building.
The Associated Press contributed to this report.