Police are likely examining critical evidence from the scene, notably shell casings and bullets

 Investigators are likely analyzing key pieces of evidence related to the weapon used to kill the CEO of UnitedHealthcare on Wednesday morning.

Shell casings and the bullets left behind at the scene can give investigators crucial information, CNN security correspondent Josh Campbell said.

“It’s essentially like a fingerprint,” Campbell said. “Firearms leave individual unique markings on both the bullet and the shell casing.”

CEO Brian Thompson was walking to the Midtown hotel when the gunman approached from behind and shot him at least once in the back and once in the right calf, NYPD commissioner Jessica Tisch said earlier Wednesday. Police said the shooter fired multiple shots, even unjamming the gun at one point, before firing again.

In addition to NYPD, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) might also be assisting with identifying the markings on the casings and the bullets, Campbell said.

Officials then run their findings through a database to determine if it is a firearm that was used in a past crime, Campbell said. That is something investigators will want to rule out.

How it works: The AFT uses specialized equipment to produce high resolution, 3D images of the headstamp and extraction marks on a shell casings.

The headstamp refers to the indentation a gun’s firing pin leaves on the end of a shell casing as a bullet is fired. Extraction marks are the result of metal-on-metal friction as a gun ejects an empty ammunition cartridge after the trigger is pressed.

The examiner then uploads the digital images into the ATF database and another examiner begins comparing the images to other shell casings recovered from crime scenes. If a computer algorithm identifies a potential match, the examiner will overlay the two images to further compare each shell casing’s microscopic features, ATF special agent Sam Ward told CNN previously.