Police in New York have released two photos of an unmasked individual wanted for questioning over killing of a healthcare chief executive.

UnitedHealthcare boss Brian Thompson, 50, was fatally shot in the back on Wednesday morning outside the Hilton hotel in Midtown Manhattan.

The attacker fled the scene without taking any of Thompson's belongings. Police believe the victim was targeted in a pre-planned killing.

Investigators are also using facial recognition technology and bullet casings with cryptic messages written on them to track down the suspect. They have yet to reveal a motive in the shooting.

Here's what we know about the suspect and the investigation.

The shooting took place at about 06:45 EST (11:45 GMT) in a busy part of Manhattan close to Times Square and Central Park. Thompson had been scheduled to speak at an investor conference later in the day.

According to police, the suspect - who was clad in a black face mask and light brown or cream-coloured jacket - appeared to be waiting for Thompson for five minutes outside the Hilton hotel where he was expected to speak.

Thompson, who arrived on foot, was shot in the back and leg, and was pronounced dead about half an hour later at a local hospital.

New York Police Department (NYPD) Chief of Detectives Joseph Kenny has revealed that the suspect's weapon appeared to jam, but that he was able to quickly fix it and keep shooting.

CCTV footage appears to show the gunman had fitted a suppressor, also known as a silencer, to his pistol, BBC Verify has established.

Video shows him fleeing the scene on foot. Officials initially said the suspect used an electric Citi Bike owned by Lyft.

But Lyft, which owns and operates Citi Bike, later said it had been told by the NYPD that one of its vehicles had not been used, according to the BBC's US partner, CBS News.

So far, the investigation into Thompson's killing has centred on a few clues that police are using to identify the suspect.

Officials released two images of an unmasked man on Thursday that the NYPD said was "wanted for questioning" in connection with the murder.

It is unclear if he is the same person as the suspect.

Earlier, police revealed the suspect was photographed at a nearby Starbucks just minutes before the shooting.

While he is masked in the image, police sources told CBS that the mask is pulled down far enough so that his eyes and part of his nose can be seen.

With that, investigators are using facial recognition software to try to find a match.

Investigators have so far not identified a motive in the killing, although police did note that the assailant fled without taking any of Thompson's belongings.

Additionally, police are testing three bullet casings and three live rounds found at the scene for DNA.

The words "deny," "defend" and "depose" were discovered on the casings, two law enforcement sources told CBS.

Investigators believe this could be a reference to the "three D's of insurance" - a known reference made by opponents of the industry.

The terms refer to tactics used by insurance companies to refuse payment claims by patients in America's complicated and mostly privately run health care system.

A mobile phone was discovered in an alley along the suspect's escape route. Police say they are "working through" the phone.

Investigators also said they executed a search warrant at a location in Manhattan's Upper West Side, which he was seen entering earlier in the day.

The location is near the Frederick Douglas housing project, where police say surveillance video showed the suspect outside at approximately 5 AM the morning of the crime.

Police earlier said they would also search Thompson's room at the nearby Marriott, which is down the street from where the incident took place.

Thompson joined UnitedHealth, the biggest private insurer in the US, from accountancy firm PricewaterhouseCoopers in 2004.

He rose through the ranks and became CEO in 2021, leading the company through some very profitable years.

In an interview with MSNBC, Thompson's wife said that there had "been some threats" against him earlier, although she was unable to provide details.

"I just know that he said there were some people that had been threatening him," she said.

According to police in Thompson's hometown of Maple Grove, Minnesota, there had previously been one suspicious incident at his home in 2018.

The incident was cleared with no criminal activity detected. No additional details were provided.