NEW YORK, Dec 7, (AP): The gunman who killed the CEO of the largest US health insurer likely left New York City on a bus soon after the brazen ambush that has shaken corporate America, police officials said. But he left something behind: a backpack that was discovered in Central Park. Nearly four days after the shooting of UnitedHealthcare CEO Brian Thompson, police still did not know the gunman’s name or whereabouts or have a motive for the killing.

Investigators were looking at whether the shooter may have been a disgruntled employee or client of the insurer, Chief of Detectives Joseph Kenny told reporters. The FBI announced Friday night it was offering a $50,000 reward for information leading to an arrest and conviction. Video of the gunman fleeing Wednesday’s shooting showed him riding a bicycle into Central Park and later taking a taxi to a bus terminal that offers commuter service to New Jersey and Greyhound routes to Philadelphia, Boston and Washington, DC according to Kenny.

Police have video of the man entering the bus station but no video of him exiting, leading them to believe he left the city, Kenny said. Investigators on Friday found a backpack in the park that had been worn by the gunman during the shooting, police said, following a massive sweep to find it in a vast area with lakes and ponds, meadows, playgrounds and woods.

Police didn’t immediately reveal what, if anything, it contained but said it would be analyzed for clues. The gunman made sure to conceal his identity with a mask during almost all of his time in the city, including during the attack and while he ate, yet left a trail of evidence in view of the nation’s biggest city and its network of security cameras.

The gunman arrived in New York City on Nov 24 and shot Thompson 10 days later outside his company’s annual investor conference at a hotel just blocks from Radio City Music Hall and Rockefeller Center. The gunman got off a bus that originated in Atlanta and made several stops along the way, Kenny said. Police have not determined where he got on the bus.

Investigators have a list of passengers, but none of them would have had to provide an ID when they climbed aboard, Kenny said. Investigators believe the suspect used a fake identification card and paid cash, Kenny said, when he checked in at the hostel, which has a café along with shared and private rooms and is blocks from Columbia University.