BRIAN THOMPSON SHOOTINGALLEGED KILLER TRAVELED TO NYC BY BUS FROM ATLANTA

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.