By Hollie Silverman, Darran Simon and Brynn Gingras, CNN

A typical afternoon inside the offices of a Midtown Manhattan skyscraper suddenly turned to chaos Monday when a helicopter, 11 minutes into its flight, crash-landed on the roof.

Floors of the building shook. Before the alarms started to blare and workers understood what was happening, security was ordering people to grab their belongings and evacuate out into the rainy, foggy streets of New York.

Frantic employees squeezed into the stairwell, hurrying down flights, unaware that a helicopter had crashed on top of their building, leaving one person dead.

"It took a half hour to get from the 29th floor down to the ground floor. There were just too many people; it was too crowded, and everybody was trying to get off on all the floors at the same time," Nathan Sutton said, standing outside 787 Seventh Ave.

"You could feel the building shake, and you could actually hear the alarms," he said.

The pilot, identified as Tim McCormack, died in the crash, law enforcement said.

City officials breathed a sigh of relief that no was injured inside the building or on the streets below. Yet they said Monday afternoon they were not sure what led the pilot to crash-land atop a building without a helipad.

Based on interviews investigators conducted at the East 34th Street Heliport on Manhattan's East Side, the pilot made statements that he believed he had a 5-to 7-minute break in the rainy weather to take off, according to a law enforcement source familiar with the investigation. The pilot did not refuel at the heliport, the source added.

Once the pilot was in the air, he radioed back to the heliport and said he needed to return. The last time the pilot communicated with the heliport he conveyed he was unsure of his location, the source said.

'My mind goes where every New Yorker's mind goes'

Lance Koonce was one block away from 787 Seventh Ave. when he heard something that sounded like a helicopter flying low. He saw a sheet of flames and smoke when he looked out the window.

Morgan Aries was inside the crash site on the 14th floor.

"We felt a little bit of a tremor," he told CNN.

The order to evacuate came minutes later, he recalled.

"There was a moment in which we all couldn't get out of the building because we're all just backlogged in there," Aries said.

Gov. Andrew Cuomo was among the New Yorkers who said the crash brought back memories of the September 11 terror attacks at the World Trade Center in Lower Manhattan.

"If you're a New Yorker, you have a level of PTSD from 9/11," Cuomo said. "And I remember that morning all too well. So as soon as you hear an aircraft hit a building, my mind goes where every New Yorker's mind goes."

Despite the scare, inspectors and engineers found no evidence of structural damage to the tower and determined the building does not pose a danger to the public, according to Andrew Rudansky, senior deputy press secretary for the New York City Department of Buildings.

Fighting the fire

The helicopter took off from East 34th Street Heliport about 1:32 p.m. Monday, New York police Commissioner James O'Neill said, and it crashed about 11 minutes later.

At the time, moderate to heavy rain was falling in the city, and visibility at Central Park was down to 1.25 miles. Winds were from the east at 9 mph.

The pilot then flew around Battery Park on the southern tip of Manhattan, up the west side of the island and then, somewhere around the streets in the 40s, started to veer toward Midtown Manhattan before crash-landing, the law enforcement source said.

O'Neill could not say whether the pilot made an emergency call from the Agusta A109E helicopter.

The first firefighters were on the scene within five minutes, Thomas Richardson, chief of fire operations, told reporters. Firefighters climbed to the top of the 54-floor building to put out the three-alarm fire.

Lt. Adrienne Walsh, one of the fire department's first responders, described the roof scene as "a debris field that was on fire."

Mourning a pilot, a volunteer firefighter

McCormack had flown for American Continental Properties, the company that owns the helicopter, for five years, according to a company statement.

"We are mourning the loss of Tim McCormack," it said.

McCormack received his commercial pilot's license in 2004, according to Federal Aviation Administration records, and he was certified as a flight instructor for a rotorcraft-helicopter last year.

In October 2014, the pilot was flying a helicopter over the Hudson River with six tourists on board when a bird struck and broke part of the windshield, according to CNN affiliate WABC.

McCormack was forced to make an emergency landing at the West 30th Street Heliport. No one was injured, according to the report.

McCormack said at the time that it was "pretty much like an explosion going off in your cockpit."

His passengers started screaming and crying, the station reported.

"A little bit of pandemonium," he told the station, recalling the incident. "You kind of gather yourself, and we headed over till we landed at 30th Street."

McCormack had volunteered with the East Clinton Volunteer Fire Department since 1994 and was the department chief for 10 years, East Clinton Fire Department Chief Don Estes said. McCormack had also volunteered with the LaGrange Fire Department, according to Estes.

"Tim was a dedicated, highly professional and extremely well-trained firefighter," Estes said reading a statement. "Tim's technical knowledge and abilities to command an emergency were exceptional."

McCormack was respected by his department and other firefighters in Dutchess County, he said.

"Tim will be exceptionally missed by his department members, not only for his leadership but his wonderful sense of humor,' Estes said.

"Rest in Peace, brother."

The investigation

The National Transportation Safety Board released details Tuesday about its investigation, which could take as many as 24 months to finish, though investigators are aiming for 15 months. A preliminary report is expected in two weeks, but it will not provide a cause for the crash.

The helicopter had no flight data recorder or voice recorder, nor was it required to have those devices on board, said NTSB air safety investigator Doug Brazy. The aircraft did, however, contain other devices for recording data, and investigators continue to search for them, he said.

Complicating matters is that the crash debris is located on a rooftop. "It's highly fragmented and a post-crash fire consumed much of the evidence," he said.

The pilot was never in communication with air-traffic control, but that was not a requirement either, Brazy said.

The NTSB hopes to finish the portion of its investigation related to the rooftop sometime Tuesday, he said, and will then move the debris to another location for further inspection.

Asked how a salvage company working for the NTSB plans to remove the wreckage from the roof, Brazy said, "It may be down the stairs and down the elevator."

Investigators are interviewing witnesses, including heliport employees and a passenger who completed a previous flight on the helicopter and noticed nothing out of the ordinary, Brazy said. They're also conducting a toxicology test on the slain pilot, which is standard procedure, and trying to determine what role weather played in the crash.