The killer arrived first. He stood in the cold predawn gloom outside the New York Hilton Midtown and waited. Even at that early hour, people passed by. He ignored them. They ignored him.

At 6:44 a.m., he saw his man. Brian Thompson, 50, chief executive of UnitedHealthcare — the leader of one of the country’s largest companies — walking past in a blue suit toward the entrance to the Hilton.

It was the site of press events and celebrity galas dating back to Elvis Presley and Ronald Reagan. On Wednesday, it was where UnitedHealthcare was holding its annual investors day, and within an hour it would be filled with Wall Street analysts and stockholders.

The killer popped out from behind a car and raised a pistol fitted with a long silencer. What followed was what the police would call a bold assassination, which shook the insurance industry and sent a jolt through an area packed with holiday tourists.

By nightfall, a sprawling manhunt with police officers, dogs and drones spread citywide, bearing down on surveillance videos, a dropped cellphone and even Citi Bike data in search of the killer.

The police commissioner, Jessica Tisch, who was sworn in 10 days ago, called it a “brazen targeted attack,” adding, “We will not rest until we identify and apprehend the shooter in this case.”

The killer wore a dark hooded jacket and a gray backpack, pictures released by the police show, with his face covered to his nose. He apparently knew which door Mr. Thompson was going to enter and arrived outside the hotel about five minutes earlier.

Surveillance video shows Mr. Thompson’s arrival. The shooter, seen from behind, walks up and fires at least three times, striking Mr. Thompson in the calf and in his back.

The victim manages a couple of steps and turns to face his attacker before collapsing on the sidewalk. The shooter’s pistol jammed during the shooting, but the gunman quickly cleared the jam and resumed firing, the police said.

A woman who was standing nearby flees. The shooter ignores her.

The shooter fiddles with his weapon and walks slowly toward Mr. Thompson, who is crumpled against a wall. He seems to point the gun at Mr. Thompson one last time, then walks away. He breaks into a run only as he crosses the street.

After the shooting, he cut through a pedestrian walkway to West 55th Street and jumped on a bike, pedaling north into Central Park, the police said.

The setting and method of the killing led detectives down several avenues of investigation. The hotel is one of the city’s largest, close to the Museum of Modern Art and Rockefeller Center, and the surrounding blocks are rife with private and city surveillance cameras that could show where the killer came from and where he fled, as well as images of his face.

Investigators were also pursuing leads involving the bike, which they said may have been a Citi Bike. Riders must use a debit or credit card to borrow a bicycle, and the departing and arriving docks and times are recorded. Armchair sleuths scraped Citi Bike’s data for nearby bicycle use at the time of the attack.

Officers also recovered a cellphone, and detectives were conducting a forensic analysis to see whether it was linked to the shooting, the police said.

The police were also exploring Mr. Thompson’s background for clues. He had recently received several threats, according to a law enforcement official familiar with the investigation, but their source and precise nature was unclear. Chief executive officers of health care companies often receive threats because of the nature of their work.

Mr. Thompson was promoted to chief executive of Minnesota-based UnitedHealthcare in April 2021, heading one unit of the larger UnitedHealth Group. He received a total compensation package last year of $10.2 million, a combination of $1 million in base pay and cash and stock grants.

In a statement, UnitedHealth Group said the company was “deeply saddened and shocked” by Mr. Thompson’s death. “Our hearts go out to Brian’s family and all who were close to him,” the statement said.

Mr. Thompson managed a division that offers insurance to employers and individuals. Under his tenure, UnitedHealthcare and its parent company have enjoyed profitable growth but have also been the subject of investigations into denials of authorization for health care procedures.

The shooting happened as Mr. Thompson arrived early to prepare for the investors day, the police said. The events are common for publicly traded companies, giving analysts who cover the company and large investors the chance to hear from senior executives and pepper them with questions.

Michael Ha, a stock analyst at Baird who was at the meeting, arrived shortly after 8 a.m. Andrew Witty, the chief executive of UnitedHealth Group, made prepared remarks shortly after. Then, notifications started to hit attendees’ phones that something had happened.

Initially there was confusion. Many wondered if it had been Mr. Witty that had been shot since he had just left the room — and they feared the gunman was somewhere in the hotel.

Mr. Witty returned to the stage to break the news to attendees.

Outside, the shooting sent a jolt of grim reality into seasonal festivities in the surrounding blocks. Revelers and tourists looking ahead to the evening’s Christmas tree lighting in Rockefeller Center awoke to police tape blocking the shooting scene just blocks north. The police assured those planning to attend the lighting that a huge police presence would be on hand.

The killing shocked the industry in which Mr. Thompson was a leader. UnitedHealth Group, the publicly traded parent of UnitedHealthcare, has a market valuation of $560 billion, similar in size to Visa or Exxon Mobil.

Mr. Thompson spent more than 20 years climbing through the ranks at its insurance division, which is among the nation’s largest with $372 billion in revenue last year and about 140,000 employees. There were no signs that his ascent was slowing. During his tenure, UnitedHealthcare profits rose, with earnings from operations topping $16 billion in 2023, up from $12 billion in 2021.

Mr. Thompson owned about $20 million of shares in UnitedHealth Group, as of late September, according to regulatory filings. In April, Bloomberg News reported that he was one of at least three executives at the company who had sold shares before a Federal antitrust investigation was disclosed to the company’s investors — about $15 million worth in Mr. Thompson’s case. The company told Bloomberg at the time that it had approved the sales.

Before he went to work at United, Mr. Thompson spent nearly seven years at PricewaterhouseCoopers, the large accounting firm also known as PwC. He graduated from the University of Iowa with an accounting degree in 1997.

Mr. Thompson lived with his family in a suburb of Minneapolis. He is survived by his wife, Paulette R. Thompson, a physical therapist who works for a Minnesota health system, and two children.

“Brian was a wonderful person with a big heart and who lived life to the fullest,” Paulette Thompson said in a statement. “He will be greatly missed by everybody. Our hearts are broken, and we are completely devastated by this news. He touched so many lives. We ask everybody to respect our privacy during this time.”

At UnitedHealthcare headquarters nearby, a cluster of tan-colored buildings more than 1,000 miles from the shooting, a lone police cruiser sat posted out front.

Christopher Maag, Ernesto Londoño Reed Abelson, Claire Fahy, Stefanos Chen and Ana Ley contributed reporting. Kitty Bennett contributed research.