Brian Thompson, the CEO of UnitedHealth's insurance unit, was fatally shot outside a Midtown Manhattan hotel in what police described as a "brazen" targeted attack by a gunman lying in wait for him.
The shooting occurred around 6.40am (11.40am Irish time) outside the Hilton on Sixth Avenue, just before a scheduled company investor conference at that hotel.
Mr Thompson, 50, was taken to a nearby hospital, where he was pronounced dead.
Police said the gunman remains at large and they are still investigating a motive.
"This does not appear to be a random act of violence," said New York City Police Commissioner Jessica Tisch at a press conference.
"Every indication is that this was a premeditated, pre-planned, targeted attack," she said, adding "it appears the suspect was lying in wait for several minutes".
A police poster offering a reward for information about the shooting shows surveillance stills of a man wearing a grey-coloured backpack riding an electric bike and another photo of the individual appearing to be pointing a gun.
The suspect fled on foot before mounting an electric bike and riding into Central Park, police said.
The killing took place just hours before the city's annual Christmas tree lighting at Rockefeller Center a few streets away, a televised event that draws massive crowds.
Police officials said the lighting would proceed as planned under heavy security.
Mr Thompson's wife, Paulette, told NBC News that he had received some threats, though she did not know any specifics.
"Basically, I don't know, a lack of coverage?" she said, according to the network.
"I don't know details. I just know that he said there were some people that had been threatening him."
UnitedHealth is the largest US health insurer, providing benefits to tens of millions of Americans, who pay more for healthcare than in any other country.
The company has been grappling with the fallout from a massive data hack of its Change Healthcare unit that provides technology for US health providers, disrupting medical care for patients and reimbursement to doctors for months.
The company also has an Irish office in the IFSC in Dublin.
"Brian was a highly respected colleague and friend to all who worked with him," UnitedHealth Group said in a statement.
"We are working closely with the New York Police Department and ask for your patience and understanding during this difficult time. Our hearts go out to Brian's family and all who were close to him."
Video showed the gunman arrived outside the Hilton about five minutes before Mr Thompson.
He ignored several other people walking by, NYPD Chief of Detectives Joseph Kenny told reporters.
When Mr Thompson approached the hotel, the gunman shot him in the back with a pistol and then continued firing, even after his gun appeared to jam.
"Based on the evidence we have so far, it does appear that the victim was specifically targeted, but at this point, we do not know why," Mr Kenny said.
The shooting happened not long before a scheduled investor conference at the Hilton.
UnitedHealth Group Chief Executive Andrew Witty took the stage about one hour after the event started to announce the rest of the programme would be cancelled.
"We're dealing with a very serious medical situation with one of our team members, and as a result, I'm afraid we're going to have to bring to a close the event today," he said.
Police tape blocked off the area on 54th Street outside the Hilton, where blue plastic gloves were strewn about and plastic cups appeared to mark the location of bullet casings.
"The police were here in seconds. It's New York. It's not normal here at seven in the morning but it's pretty scary," said Christian Diaz, who said he heard the gunfire from the nearby University Club Hotel where he works.
Mr Thompson was named UnitedHealthcare CEO in April 2021 after working at the company since 2004 in several departments, according to the company's website.
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"Sometimes you meet a lot of fake people in these corporate environments, he certainly didn't ever give me the impression of being one of them," said Antonio Ciaccia, CEO of healthcare research non-profit 46brooklyn, who knew Mr Thompson.
"He was a genuinely thoughtful and respectable guy," he said.
Politicians including Minnesota Governor Tim Walz and Senator Amy Klobuchar released condolence statements.
"This is a horrifying and shocking act of violence. My thoughts are with Brian Thompson's family and loved ones and all those working at United Healthcare in Minnesota," said Ms Klobuchar.
Consumer frustration with the US healthcare system runs high and was the driving force behind the US Affordable Care Act, which created a new type of health insurance and mandated coverage for preventative care as well as maximum profit levels.
