Killer carved three mysterious words into bullets used to assassinate UnitedHealthcare CEO Brian Thompson

The person who gunned down UnitedHealthcare boss Brian Thompson inscribed three words on the bullets used to kill him, police sources have said.

Thompson was shot and killed yesterday morning (December 4) at 6.46am Eastern Time, outside the Hilton Hotel in Manhattan, New York City.

The father-of-two was shot at least once in the back and his right calf and was rushed to Mount Sinai Hospital in a critical condition, but was later pronounced dead upon arrival.

According to the New York Police Department (NYPD), the gunman appeared to have been 'lying in wait' for Thompson before shooting him, arguing that this attack does not appear to be a random act of violence'.

Cops also believe that the suspect, who as of this morning (December 5) is still on the run, fled the scene of the crime by running down an alley before witnesses saw him hopping on a electric bike and making his escape through Central Park - where he was last seen at 6.48am.

Amar Abdelmula witnessed the assassination unfold from inside his car, telling PIX11: "I wasn't paying attention and then I heard the shot.

Police are looking for the suspect who shot Brian Thompson (United Healthcare Group)

"It was [a] silent gun, black gun, [I] saw him after he shot him and was running across the street. I tried to take a picture, but [he was] too far away, not clear.

"I was shocked. First time I see crime in front of me. I was afraid he (would) shoot me too, my car, I saw everything."

Now, the New York Post and ABC News have reported that sources at NYPD supposedly confirmed that three words were inscribed on the bullets - 'deny', 'depose' and 'defend'.

The words were engraved on the shell casings and live rounds fired at Thompson, which The New York Post highlighted is similar to a book from 2010 that condemned the insurance industry.

Titled Delay, 'Deny, Defend', the book's blur reads: "Why insurance companies don't pay claims and what you can do about it."

United Healthcare was said to be holding an investor conference on Wednesday (December 4), but that was quickly canceled following the shooting.

The suspect was pictured at Starbucks minutes before gunning down the father-of-two (NYPD)

Andrew Witty, CEO of United Healthcare's parent company UnitedHealth Group, said at the event: "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."

While a potential motive is yet to be determined by police, Thompson's wife, Paulette Thompson, also spoke to NBC News where she revealed that there had 'been some threats' made against her husband, adding: "I don’t know details. I just know that he said there were some people that had been threatening him.

“I can’t really give a thoughtful response right now. I just found this out and I’m trying to console my children.”