UnitedHealthcare CEO killer sent a 'chilling' message, former FBI agent says

Evidence shows that each bullet and casing had a single word on it

As the manhunt for UnitedHealthcare CEO Brian Thompson's killer continues, one crime expert is spotlighting a "highly disturbing" detail that has emerged from the investigation.

New evidence at the scene in Midtown Manhattan reveals that the words "deny," "depose" and "defend" were written on the live rounds and casings — a "chilling" new detail, according to former FBI special agent Nicole Parker. 

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"Take nothing off the table at this point because we're just at the very beginning stages of this investigation. But those shell casings, that is chilling," former FBI special agent Nicole Parker said Thursday.

"And shell casings, people don't really think about that. But there's a lot of evidence that can come out of shell casings and the fact that there were words inscribed on them, that is very, very telling."

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In a shocking video obtained by Fox News Digital, Thompson is seen walking down a New York City sidewalk when a masked man wearing a black hooded jacket and a backpack walks up behind him and raises a handgun.

Following the shooting, the suspect walked between two vehicles, got onto a bike then disappeared into Central Park, the New York City Police Department (NYPD) said.

The former FBI agent spotlighted another piece of potentially case-changing evidence. According to reports, an abandoned cellphone was found in an alley.

"The phone that was found in the alley, that is a treasure trove of information, if that was indeed the murderer's phone," Parker explained. 

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The former FBI special agent continued, revealing the "most important" part of investigators' strategy.

"The most important thing is that this person is identified and that he is brought into custody…. Somebody knows this shooter, somebody knows that this individual did this, and they need to report it to law enforcement," Parker argued Thursday.

"Right now, there is a cash reward. I think they should up the cash reward substantially. But I do believe that someone knows who this individual is. And that is the greatest hope, in my opinion, of getting cooperation from the public to give as many tips as possible to lead them."

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Anyone with information is encouraged to contact Crime Stoppers at 1-800-577-TIPS, or for Spanish, 1-888-57-PISTA. Tips can also be submitted on the Crime Stoppers website by visiting crimestoppers.nypdonline.com.

Fox News' Christina Shaw contributed to this report.