A helicopter crashed onto the fog-shrouded roof of a midtown Manhattan skyscraper on Monday, killing the pilot and unnerving a city still scarred by memories of the Sept. 11, 2001, airplane attacks on the World Trade Center.

The crash on a rainy, gray day atop the 54-storey AXA Equitable Center forced office workers to evacuate in one of the city's busiest areas a few blocks north of Times Square.

The pilot was the only person aboard the chopper when it plunged into the building and burst into flames, New York City Mayor Bill de Blasio told a news conference. No injuries, either to people in the building or on the ground, have been reported.

"The helicopter is pretty obliterated at this point. It was obviously a very hard hit," de Blasio said, adding nothing indicated "an act of terrorism."

Nicolas Estevez was standing across the street from the building when a 12-inch (30 cm) piece of metal that appeared to be from the helicopter landed on the pavement just feet away.

The crash, which sent people streaming out of the building within seconds, reminded him of Sept. 11, Estevez said.

"I saw the explosion and the smoke coming out," he said.

KEY MYSTERY

A key mystery in the crash is why the Agusta A109E was flying at all in a rainstorm in tightly controlled airspace above midtown Manhattan.

To enter that vicinity, de Blasio said, the pilot would have needed approval from the air traffic control tower at LaGuardia Airport across the East River in Queens, “and we need to find out if that happened.”

The Federal Aviation Administration said in a statement “FAA air traffic controllers did not handle” the helicopter’s flight, but a spokeswoman for the agency declined to say whether the aircraft was observing prevailing flight restrictions.

The pilot was identified as Tim McCormack, who was going to land at Linden Airport in New Jersey, said Paul Dudley, the airport's director.

“Tim McCormack is a well-respected, highly trained veteran pilot who also had tremendous local knowledge, having flown in this area for many years,” Dudley said in a phone interview. “We’re all saddened and shocked.”

McCormack worked for Daniele Bodini, founder of the real estate firm American Continental Properties Group, Dudley said.

The chopper took off from a heliport on Manhattan's east side at 1:32 p.m. and crash-landed on the building 11 minutes later, officials said.

The site is about half a mile from Trump Tower, where US President Donald Trump maintains an apartment. The area has been under a temporary flight restriction since his election in November 2016.

FELT LIKE HE WAS SHOVED

Nathan Hutton, who works in information technology for the French bank BNP Paribas on the 29th floor, said the building shook when the helicopter slammed into the roof.

"It felt like you were just standing there, and someone takes their hand and just shoves you," he said. "You felt it through the whole building."

Trump called New York Governor Andrew Cuomo, who was at the scene soon after the crash, to offer assistance if needed, the governor's office said.

Cuomo said the crash likely stirred memories of Sept. 11 for many city residents.

"If you're a New Yorker, you have a level of PTSD from 9/11," he said.

In addition to BNP Paribas, the AXA Equitable Center, built in 1985 houses offices for corporate tenants such as law firms Willkie Farr & Gallagher and Sidley Austin, and investment manager New Mountain Capital. Le Bernardin, one of New York City's most celebrated restaurants, is also in the building.

The skyscraper is managed by Los Angeles-based CommonWealth Partners. CommonWealth office manager LeAnn Holsapple said the company had "no comment at this time."

Manhattan has only three approved heliports after they were banned in New York City in 1977 after a rotor blade snapped on a helicopter on top of the former Pan Am building, killing five people.

POSSIBLE FLIGHT RULE VIOLATIONS UNDER SCRUTINY

The deadly rooftop crash-landing raised immediate questions about whether the pilot, the lone fatality, violated rules governing midtown Manhattan's tightly restricted airspace.

Mayor Bill de Blasio said the pilot, whose helicopter crashed on top of a 54-story office tower on Seventh Avenue and burst into flames, would have needed approval from air traffic controllers at nearby LaGuardia Airport to fly in that vicinity.

The privately owned aircraft for executive charter flights went down about 11 minutes after taking off from a heliport on East 34th Street, apparently en route to its home base in New Jersey, city Police Commissioner James O'Neill said.

Asked by reporters why the pilot was out flying through midtown Manhattan in the rain and low visibility, O'Neill said, "We're not sure. That's part of the investigation."

De Blasio said authorities were seeking to determine whether the pilot made contact with LaGuardia's tower or other air traffic controllers before the crash.

FLYING BELOW 1,100 FEET

The rules allow a chopper following the East River to operate free of air traffic control by staying below an altitude of 1,100 feet (335 meters). If the helicopter climbs above that height, then LaGuardia tower is supposed to assume control of the flight, according to the rules.

FAA spokesman Kathleen Bergen declined to say whether the helicopter that crashed was flying below 1,100 and whether it was following the East River corridor, or whether additional restrictions might apply in low visibility, saying those issues remained under investigation.

The crash investigation is to be led by the National Transportation Safety Board.

The crash site is about a half mile from Trump Tower, where US President Donald Trump maintains an apartment, an area that has been under extra-tight flight restrictions since Trump's November 2016 election.

De Blasio stressed no evidence pointed to an act of terror or criminal intent.

There is no landing pad on the building, the mayor said. Helipads have generally been banned from Manhattan rooftops under rules imposed following a fatal 1977 helicopter crash at an office tower then known as the Pan Am building.

In 2016, bowing to complaints about noise and air pollution, the city reached a deal with Manhattan's helicopter sightseeing tour operators to cut the number of their flights by half.

According to the New York Times, those flights numbered more than 59,000 in 2015, all of them departing from Pier 6 near the Battery at the southern tip of Manhattan, the only heliport providing the tours.