There has been a lot of speculation that the gunman who murdered UnitedHealthcare CEO Brian Thompson outside a Midtown Manhattan Hilton hotel last week was a professional killer who used a British bolt-action, magazine-fed suppressed pistol known as a "Welrod," or possibly a modern-day Welrod-variant known as the Brügger & Thomet (B&T) VP9 pistol.

Firearms expert David Katz, a former DEA firearms instructor who is now the CEO of Global Security, told Fox News that the killer might have used Welrod and/or a "modernized version of a World War II pistol." 

Welrod

"The operation that he does with his hands is consistent with the operation of that weapon," Katz said, noting that he "immediately moved to rack the slide manually with his left hand" after he fired.

B&T VP9

Multiple law enforcement sources also told Fox News that they believe the killer used a Welrod pistol. 

"I'd bet my pension that this is the weapon that was used on the United CEO. It's very, very quiet and requires manual cycling after each round is fired. Top choice by pros for up-close, quiet work," the source said.

However, firearms collector and expert Texas Gun Vault shared his opinion on the firearm used in the murder of the CEO, stating that he does not believe it was a Welrod or any variant thereof. 

"Well based off the footage that we saw of the assassination ... the assassin kept his right hand on the pistol grip and manipulated the gun with his left. So that's not the typical way that you would run a Welrod," the gun collector and expert said around the 10:30 minute mark. 

He said the "way the gun looks in profile ... does not look like a Welrod ... and I know lot of people were hoping and wishing it was because it's a cool design and it's the gun if he was some type of super secret assassin or something of that nature," adding, "Unfortunately, I don't think that's the case and the actual answer is much more mundane: What do I think it was. I believe it was a typical semi-automatic pistol with a silencer."

To save readers time, we started the video at the 10:30 mark.

Listen for about five minutes as the gun collector provides insightful commentary on what he believes the firearm used and explains how those unspent 9mm bullets with messages ended up at the crime scene.