UnitedHealthcare CEO Killer's Bullets Had Words Engraved on Shell Casings

The gunman who fatally shot UnitedHealthcare CEO Brian Thompson on the bustling streets of NYC carved a suspected message into the shell casings of the bullets he used to execute the executive -- and it might offer a clue into why he did it, TMZ has learned.

Law enforcement sources tell us ... three 9MM shell casings cops recovered from the scene outside the Hilton Hotel in Midtown Manhattan had the words “deny,” “depose” and “defend” scrawled on them.

Our sources say two of the words appear in the title of a 2010 book, "Delay, Deny, Defend," which slams the health insurance sector -- and, as we now know, Thompson was one of the industry's bigwigs.

We're told detectives believe the words on the shell casings could be a window into why the assailant committed the murder.

Our sources say the suspect or one of his relatives might have been denied coverage by Thompson's company for a medical procedure. However, our sources also say he could be a disgruntled ex-employee ... or a hitman hired by someone for some other reason.

NYPD investigators are looking into all possibilities while poring over hours of surveillance video from the multitude of police cameras dotting Manhattan. Sources tell us detectives have also received Crimestoppers tips -- and are following up on some good leads.

Currently, the unidentified killer remains at large.

As we reported ... the suspect walked up behind Thompson outside the Hilton on Sixth Avenue and pumped several bullets into him, before fleeing.