Lookalikes, a message on a bullet and Monopoly money: The bizarre murder gripping America.

Brian Thompson led one of the biggest health insurers in the United States. But for those outside the industry, he was an unrecognisable name — until now.

On Wednesday, Brian went to work at UnitedHealthcare at the New York Hilton Midtown in Manhattan, where the company was hosting its annual investor day.

As he walked towards the hotel, Brian was unaware a gunman "was lying in wait". Dressed in black and covered up to his nose, the killer bode his time until the moment presented itself.

Then, just before 7am local time, he opened fire, shooting the executive twice with a handgun fitted with a silencer, AP reports.

As Brian collapsed to the floor, the gunman continued shooting then fled the area. Brian was rushed to hospital but couldn't be saved.

Authorities believe this was a targeted killing and are scrambling to piece together clues in their hunt for the gunman.

Almost one week on, authorities say the "net is tightening" on the killer. Still, more questions than answers remain as to why Brian was the victim.

Here's what we know so far.

Watch: UnitedHealthcare CEO Brian Thompson gunned down in New York. Post continues below.

Who was Brian Thompson, the UnitedHealthcare CEO murdered in the Manhattan shooting?

Brian worked as the chief executive of insurance giant UnitedHealthcare.

Brian was one of the company's highest-paid executives, with a $US10.2 million ($A15.8 million) annual compensation package, AP reports. Despite this, he kept a low profile and was not a well-known name beyond the health insurance space. UnitedHealth Group Inc's CEO Andrew Witty took on a more public-facing role in the company.

Brian leaves behind wife Paulette and two sons, aged 19 and 16.

The couple had been living in separate homes in Maple Grove, Minnesota for years, the Wall Street Journal reports.

Brian purchased his separate $US1 million home near his family in 2018, according to public real estate records, per the New York Post

Following her husband's death, Paulette issued a statement saying: "Brian was an incredibly loving, generous, talented man who truly lived life to the fullest and touched so many lives."

"Most importantly, Brian was an incredibly loving father to our two sons and will be greatly missed."

UnitedHealth Group CEO Andrew Witty addressed the shooting in a video to employees, saying there were "no words" to describe how everyone was feeling.

"There are no words to describe how so many of us are feeling right now," he said in the video obtained by CNN. "Brian was a truly extraordinary person who touched the lives of countless people throughout our organisation and far beyond. It's a terrible tragedy and I know we are in shock and disbelief," he said, per CNN.

In a public statement, the company said: "We are deeply saddened and shocked at the passing of our dear friend and colleague Brian Thompson, the CEO of UnitedHealthcare. Brian was a highly respected colleague and friend to all who worked with him."

"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."

Police place bullet casing markers outside of a Hilton Hotel in Midtown Manhattan where United Healthcare CEO Brian Thompson was fatally shot on December 4 in New York City.

What do we know about the shooting?

Police say Brian's killing was a "brazen, targeted attack".

The CEO was staying at another nearby hotel before he headed solo to the Hilton to speak at UnitedHealth Group's investor conference.

"Speaking to other employees that travelled with him to New York, it doesn't seem like he had a security detail," NYPD Chief of Detectives Joseph Kenny told reporters.

"He left the hotel by himself, was walking, didn't seem like he had any issues at at all."

About 6.40am local time on Wednesday, the gunman hung out in the freezing temperatures as he waited for his target.

"Many people passed the suspect, but he appeared to wait for his intended target," NYPD Commissioner Jessica Tisch told reporters.

"I want to be clear: At this time, every indication is that this was a premeditated, preplanned, targeted attack."

About five minutes later, Brian arrived. It was the moment the gunman had been waiting for.

He ignored other pedestrians and approached the CEO from behind and shot him at least once in the back and once in the right calf, Tisch said.

After the initial shots, his 9mm pistol jammed but he quickly fixed it and kept firing, Kenny said, leading police to believe he is proficient in the use of firearms.

The assailant then ran from the area before fleeing on an e-bike into the nearby Central Park.

Police believe he left the city by bus soon after and may have travelled interstate. He was seen on video at an uptown bus station about 45 minutes after the attack, Kenny said.

Brian was rushed to Mount Sinai West in a critical condition. He died at 7:12am local time.

Hours after the cold-blooded murder, bomb threats were made to the homes of Brian and his widow, CBS News reports.

"No devices or suspicious items were located during the investigation," the local Maple Grove Police Department said. "The case is considered an active investigation, while the incident appears to be a hoax."

According to the New York Post, a security guard and UnitedHealthcare publicist were stationed outside Paulette's house on Thursday local time.

One neighbour told the publication the guard had been posted there throughout the night.

🚨UPDATE: Below are photos of a person of interest wanted for questioning regarding the Midtown Manhattan homicide on Dec. 4.

The full investigative efforts of the NYPD are continuing, and we are asking for the public's help—if you have any information about this case, call the… https://t.co/U4wlUquumf pic.twitter.com/243V0tBZOr

— NYPD NEWS (@NYPDnews) December 8, 2024

A bizarre lookalike competition.

While celebrity lookalike trends have been making headlines worldwide, a group of New Yorkers made the controversial move to host one mockingly celebrating Brian's assassination.

Eight contestants donned hoods and face masks as they dressed up as the killer for the contest in Washington Square Park on Saturday local time.

Dozens watched on and cheered for their winner, who took home $US50.

The winner, whose green jacket and face covering matched released images of the killer, said he headed to the park after hearing about the contest that morning.

"I got no looks until I got over here and now everyone wants a picture with me," he told the New York Post.

The contestant said the outfit didn't require much effort, explaining, "I wear this everywhere".

Another entrant had a handwritten sign over their black clothing that read, "deny, defend, depose" — the three words scrawled on the shell casings left by the gunman.

A UnitedHealthcare CEO murderer lookalike competition was held today in New York. pic.twitter.com/jtvBQWBpxv

— Pop Base (@PopBase) December 7, 2024

What was the motive?

It's the question on everyone's lips.

Former FBI supervisor Rob D'Amico said the killing had all the makings of a personal vendetta tied to UnitedHealthcare.

Speaking to NBC, he said it would have been easy for someone to find out Brian's whereabouts. Someone simply needed to make a few quick Google searches to figure out he would be at the conference and track down its location.

"Even a layman can find there's so much on the internet now and people put so much out there," D'Amico said.

And there could be some weight to this theory.

The shooter reportedly used ammunition marked with the words "deny," "defend" and "depose".

Police are exploring whether this is linked to a popular phrase in the insurance industry: "delay, deny, defend".

The slain CEO had been receiving recent threats but hadn't been altering his travel routine, according to his wife.

"Yes, there had been some threats. Basically, I don't know, a lack of coverage? I don't know details. I just know that he said there were some people that had been threatening him," Paula told NBC News.

Investigators at UnitedHealthcare are now combing through files looking for a possible motive, any threat and disgruntled or recently fired workers, a source told CBS News.

Police are also scouring social media and talking to employees, family and acquaintances of Brian, Chief Kenny said.

New York police have released photos of a person of interest in the shooting of Brian Thompson. Image: NYPD.

Who was the shooter?

At this stage, details on who killed Brian are light, but authorities say they're closing in on his arrest.

"The net is tightening," New York City mayor Eric Adams told reporters.

However, Adams declined to confirm if the suspect had been identified.

"We don't want to release that now. If we do, we are basically giving a tip to the person we are seeking and we do not want to give him an upper hand at all," he said, per the New York Post.

"Let him continue to believe he can hide behind the mask. We revealed his face. We're going to reveal who he is and we're going to bring him to justice."

Authorities believe he acted alone.

Police used dogs and drones to search for the man, as detectives combed through data related to e-bikes after the suspect used one to flee the area into nearby Central Park.

Authorities said they found a water bottle and protein bar wrapper, believed to be bought by the suspect minutes before the attack, in a rubbish bin near the scene of the shooting, per AP. The city's medical examiner is testing them.

A major search of Central Park also discovered a backpack believed to have been ditched by the killer. NBC reports there was a jacket and fake Monopoly money inside.

Divers spent Saturday searching the waters of Central Park for the murder weapon, while investigators travelled to Atlanta to find video of the suspect, who is believed to have arrived in New York on a bus from Atlanta, per NBC.

On Thursday local time (Friday morning AEDT), the New York Police Department released photos calling on the public to help them identify a "person of interest" who was wanted in connection with the shooting.

The images were taken by security cameras at a hostel on Manhattan's Upper West Side where police believe the suspect stayed.

In one, a young man wearing a dark coat with a grey hood is seen smiling.

Police have since released additional images of the suspect wearing a blue medical mask and black jacket.

The gunman is now on the FBI's Most Wanted list, with the bureau on Friday announcing it was offering a $50,000 reward for any information that leads to an arrest and conviction. This is on top of the $10,000 on offer by the NYPD.

Feature image: UnitedHealthcare and Twitter/@PopBase.