UnitedHealthcare CEO Murder Suspect Has Fled New York City, Police Believe

3:22 PM PT -- According to CNN ... an NYPD spokesperson says a backpack found in Central Park is being investigated as possibly belonging to the suspect wanted for the murder of UnitedHealthcare CEO Brian Thompson.

As TMZ reported yesterday, the backpack appears to be an Everyday Backpack from Peak Design ... a bag meant for serious photographers but designed for casual use as well.

The manhunt for the suspect who fatally shot UnitedHealthcare CEO Brian Thompson in Manhattan is expanding beyond New York City ... because cops think he fled the Big Apple.

New York City Police Commissioner Jessica Tish told CNN that NYPD investigators have reason to believe the suspect is no longer in town.

As we reported ... police believe the gunman arrived in NYC last month on a bus from Atlanta and shacked up in a hostel, using a fake ID to check in, before Wednesday's shooting outside a Manhattan hotel.

Police say the shooter made his initial getaway on a bicycle, which was captured on surveillance cameras, and rode into Central Park.

Cops released images of the suspect's face ... but he has so far not been identified.

Meanwhile, NYPD Chief of Detectives Joseph Kenny told CNN cops believe they have footage of the suspect leaving Central Park and hailing a cab ride to a Port Authority bus center.

Kenny says police are working to figure out which bus the suspect may have gotten on, noting the buses at that station travel interstate.

Stay tuned ...