The search continues for the man who shot down a New York CEO in what police believe to have been a ‘targeted attack’.

Brian Thompson, the boss of UnitedHealthcare, was brutally killed outside the Hilton Hotel on Sixth Avenue in Midtown Manhattan by a masked man holding a gun last Wednesday (4 December).

At around 6:46 am local time, the 50-year-old was shot at least once in the back and once in the calf before he was taken to hospital and pronounced dead.

‘Deny’, ‘depose’, and ‘defend’ were engraved on the live rounds and shell casings left behind from the silent gun, thought to be a connection to a book titled Delay, Deny, Defend.

The suspected assassin fled the scene with the help of an e-bike in Central Park, ditching it around 7am near 85th Street.

Officers have been checking the park for any possible clues and found his bag there on Friday.

The NYPD has since released two additional photos of the suspected gunman that appeared to be from a camera mounted inside a taxi.

The suspected shooter in a taxi (NYPD)

The first shows him outside the vehicle, and the second shows him looking through the partition between the back seat and the front of the cab. In both, his face is partially obscured by a blue, medical-style mask.

Now, the bag which was found in the park included stacks of Monopoly money, and there is a chilling theory into why that might have been the case.

Forensic psychiatrist Dr Carole Lieberman told The Daily Beast that the reason she thinks the money was there was because the killer wanted to send a message, but forgot.

Lieberman believes the killer originally wanted to throw a stack of Monopoly bills over the body of Thompson as he lay dying in the street after being shot, but he forgot to take them out of his backpack.

The shooter’s backpack had Monopoly money in it (ABC)

“The Monopoly money conveys messages, both that UnitedHealthcare is a ripoff that steals money from patients in order to pay its executives millions, and that it is itself a monopoly,” she said.

“He may have planned to scatter the Monopoly money around the CEO’s body after he shot him, but he forgot to take the bills out of his backpack.

“This wasn’t just about taking a life.

“This was about sending a message—a deeply twisted, deeply calculated one.

“This was not an impulsive act.”

Meanwhile, police commissioner Jessica Tisch said on Friday that they ‘have reason to believe that the person in question has left New York City’.