Brian Thompson's Alleged Killer Took Bus From Atlanta To New York

The gunman who killed UnitedHealthcare CEO Brian Thompson arrived in New York City on a bus from Atlanta ... TMZ has confirmed.

Law enforcement sources tell us the suspected shooter arrived in the Big Apple 10 days before the shooting on November 24 ... taking a Greyhound bus to the Port Authority station in Manhattan.

From there, we're told the suspect checked into a hostel.

Thompson was shot dead Wednesday morning outside a hotel in what cops are calling a targeted, planned attack. Cops say the gunmen then fled the scene on a Citi Bike.

Our sources say law enforcement also interviewed a woman who worked at the hostel who told investigators she flirted with the suspect and got him to lower his mask ... which is how cops ended up with photos of the guy's face.

The motive is still unclear here ... but it's worth noting the bullet shells were inscribed with three words ... "deny," "depose" and "defend."

CNN was first to report the suspect took a bus from Atlanta to NYC.