NEW YORK (Reuters) - Armed with a growing file of clues, New York police on Friday were scouring surveillance videos and asking the public for help in their search for the masked assailant who gunned down a UnitedHealth ( UNH ) executive on a Midtown Manhattan sidewalk.

Brian Thompson, 50, the CEO of UnitedHealth's ( UNH ) insurance unit and a father of two, was shot from behind early on Wednesday in what police described as a brazen and targeted attack. It came just before the company's annual investor conference at the Hilton on Sixth Avenue.

Police have released multiple photos of the suspect that were captured by security cameras from around town during his stay in New York. They have yet to publicly identify the man, who was last seen riding an electric bicycle into Central Park.

Police offered a $10,000 reward for information leading to an arrest and conviction.

The words "deny," "defend" and "depose" were carved into shell casings found at the scene, police sources told ABC and the New York Post. A New York City Police Department spokesperson would not comment on the report.

The words evoke the title of author Jay Feinman's 2010 book critical of the insurance industry entitled "Delay Deny Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It." Feinman, a professor emeritus at Rutgers University Law School, declined to comment.

UnitedHealth ( UNH ) is the largest U.S. health insurer, providing benefits to tens of millions of Americans, who pay more for healthcare than people in any other country. Thompson joined UnitedHealth ( UNH ) in 2004 and became the CEO of UnitedHealthcare, a unit of UnitedHealth Group ( UNH ), in April 2021.

Investigators believe the suspect arrived in New York City on Nov. 24 after traveling from Atlanta by bus, CNN reported, citing unnamed law enforcement sources. The suspect checked into a hostel in the city with a fake identification and paid with cash, CNN reported.

Detectives believe the perpetrator was experienced with firearms based on how he slowly and deliberately carried out the shooting, CNN reported, citing police sources who spoke on the condition of anonymity because the investigation was ongoing.

Security video showed the shooter, wearing a hooded sweatshirt, ski mask and a gray backpack, walking up behind Thompson, raising his handgun and firing at his back. Police said the gunman arrived outside the hotel several minutes before Thompson and waited for him to walk past before firing, ignoring other passers-by. 

CNN, whose reporter John Miller is a former NYPD deputy commissioner, said police found a phone in an alley that the gunman ran through and also recovered a water bottle the shooter bought just minutes before the attack.

A fingerprint on the water bottle was too smudged to provide further clues about the shooter, the New York Times reported, citing a senior law enforcement official.

New York has one of the most advanced surveillance systems of any major U.S. city, largely built after the attacks of Sept. 11, 2001, said Felipe Rodriguez, a former NYPD detective sergeant who is now an adjunct professor at John Jay College of Criminal Justice in New York.

There are thousands of cameras in New York and all feeds can be monitored in real time as well as reviewed for previous video, aided by facial recognition software.