NEW YORK (Reuters) - New York police were searching on Thursday for the man who shot dead UnitedHealth ( UNH ) executive Brian Thompson in a brazen attack outside a Midtown Manhattan hotel and was last seen fleeing into Central Park.

Thompson, the CEO of UnitedHealth's ( UNH ) insurance unit, was shot from behind on Wednesday morning in what police described as a targeted attack by a masked assailant lying in wait. It came just before the company's annual investor conference at the Hilton on Sixth Avenue.

Investigators have not yet named a suspect and were still looking for a motive, the New York City Police Department said during a press conference on Wednesday morning. They have not provided a briefing since.

"This does not appear to be a random act of violence," New York City Police Commissioner Jessica Tisch told reporters.

"Every indication is that this was a premeditated, pre-planned, targeted attack."

Security video showed the shooter behind Thompson, 50, raising his handgun and firing at his back. Police said the gunman arrived outside the hotel several minutes before Thompson and waited for him to walk past before firing.

The suspect, wearing a hooded sweatshirt and balaclava and wearing a gray backpack, fled on foot before mounting an electric bike and riding into Central Park, police said.

Police published a number of photographs of the suspect taken from video cameras in the area, including one with the gun raised and pointed toward Thompson and another of the suspect fleeing on a bike.

Other photos captured a glimpse of his eyes, brow and the bridge of his nose as he stood in a cafe, with a caption asking the public for any help in identifying the man.

The killing took place on the morning of the city's annual Christmas tree lighting at Rockefeller Center a few blocks away, a televised event that draws massive crowds. The event went ahead as planned under heavy security.

UnitedHealth ( UNH ) is the largest U.S. health insurer, providing benefits to tens of millions of Americans, who pay more for healthcare than people in any other country. Thompson, 50, had been the CEO of UnitedHealthcare, a unit of UnitedHealth Group ( UNH ), since April 2021.

The company has been grappling with the fallout from a massive data hack of its Change Healthcare unit that provides technology for U.S. healthcare providers, disrupting medical care for patients and reimbursement to doctors for months.   

Thompson had worked at UnitedHealth ( UNH ) since 2004 in several divisions, according to a biography later removed from the company's website.

"Our hearts go out to Brian's family and all who were close to him," the company said in a statement.

The police department in Maple Grove, Minnesota, where Thompson lived, said it had no record of threats against him, and the Minneapolis Police Department said there was "no occurrence" of Thompson in its records.

The shooting happened at the outset of New York's busy holiday season, expected to bring more than 7.5 million visitors to the city, according to a local tourism bureau.