The frightening message left by the assassin after shooting UnitedHealthcare CEO in bold Manhattan assault.

Detectives investigating the murder of UnitedHealthCare CEO Brian Thompson reportedly found shell casings with the words ‘deny’, ‘defend’ and ‘depose’ inscribed on them. 

Police sources told ABC News that casings were found at the scene with the cryptic messages left on them. 

Detectives are now working to determine what the words mean and if they could possibly hint at a motive for the slaying of the 50-year-old. 

Officers are still searching for the unknown assailant who ambushed Thompson outside of the Hilton Hotel in Manhattan on Wednesday morning. 

Authorities say he fled the scene on foot before then making a getaway on an electric bike into nearby Central Park. 

Surveillance footage of the shooting shows Thompson walking alone outside of the entrance to the midtown hotel when the gunman steps into frame and opens fire. 

The masked killer draws out a large gun fitted with a silencer and shots him in the back, sending the CEO stumbling down the street as he continues to fire. 

Several bystanders were seen in the footage witnessing the horror shooting, with one woman standing mere feet away from the gunman when he opened fire. 

Witnesses said the gunman was seen waiting for some time outside the hotel before the shooting, and knew which door Thompson was going to emerge from before opening fire.

Newly appointed police commissioner Jessica Tisch said the shooter appeared to be ‘lying in wait for several minutes’ before approaching Thompson.

Surveillance grabs from a nearby Starbucks show the shooter inside the establishment prior to the incident.

Sources told CNN that the man bought two powers bars and a bottle of water from the coffee chain.

Authorities found a phone and a water bottle near the scene that they believe is the one he bought from Starbucks, and could offer them DNA.

The phone could also provide fingerprints and, if technicians can unlock it, could give them an insight into his identity.

Officers also searched Thompson’s hotel room, are interviewing his UnitedHealthcare colleagues and reviewing his social media.

Officials have yet to establish a motive in the killing but sources told ABC 7 they are investigating a possible grudge against the insurance company.  

Tisch added: ‘Many people passed the suspect, but he appeared to wait for his intended target’, and said that it did not ‘appear to be a random act of violence’. 

‘From watching the video, it does seem that he’s proficient in the use of firearms as he was able to clear the malfunctions pretty quickly,’ NYPD Chief of Detectives Joseph Kenny said. 

He was also caught on surveillance footage loitering outside and appeared to be on the phone ten minutes before he opened fire, according to The New York Times. 

Thompson had been slated to speak at an investor meeting at the hotel on Wednesday morning, in which he would announce United Healthcare’s hugely profitable 2025 financial outlook – including expected revenues of more than $450 billion. 

His schedule was widely known, and witnesses have said the suspected gunman even knew which door Thompson was going to emerge from before opening fire. 

Still, the investor conference reportedly began without interruption at 8am on the second floor of the Hilton until news of the shooting began to circulate. 

Without knowing it was Thompson who was hit, one attendee told the group that ‘someone got shot outside.’ 

CEO of parent company United Healthgroup Andrew Witty is said to have halted the meeting at the hotel just after 8am upon hearing of the tragedy. 

He told attendees: ‘We’re dealing with a very serious medical situation with one of our team members. As a result, I’m afraid, we’re going to have to bring to a close the event.’ 

A Hilton worker, who asked to remain anonymous, told DailyMail.com in the aftermath that staff have been left shaken by the public assassination on their doorstep. 

United is the biggest health insurer by market share in America. They have been the subject of protests by activists for allegedly systematically denying care for patients.

One such protest earlier this year led to the arrests of 11 people outside the United Healthcare headquarters in Minnetonka, Minnesota.

The company made headlines in February after it was subjected to a cyber-attack which cost $872 million.

Thompson’s wife, Paulette ‘Pauley’ Thompson, 51, has said her husband had been receiving ‘some threats’ before his trip to New York.

‘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,’ she told NBC News. 

Thompson was also being investigated by the Department of Justice for antitrust violations and was accused of insider trading.

The department launched a probe into whether the private company was unfairly restricting competitors and running a monopoly. 

He is survived by his wife and their two children who live in the family’s $1.5 million home in Maple Grove, Minnesota.

‘We are shattered to hear about the senseless killing of our beloved Brian,’ she told ABC 7 following her husband’s untimely death.

‘Brian was an incredibly loving, generous, talented man who truly lived life to the fullest and touched so many lives,’ she said.

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

‘We appreciate your well wishes and request complete privacy as our family moves through this difficult time.’