(Reuters) - New York City police believe the man who fatally shot a UnitedHealth ( UNH ) top executive has left the city, Police Commissioner Jessica Tisch said on Friday, as the hunt for the gunman passed the crucial 48-hour mark.

Brian Thompson, 50, the CEO of UnitedHealth's ( UNH ) insurance unit, was shot in the back on Wednesday in what police described as a targeted attack. Police have released multiple photos of the suspect - who fled the scene, climbed on an electric bicycle and disappeared into Central Park - and have asked the public for help in tracking him down.

Police on Friday recovered what they believe to be the suspect's backpack in Central Park, CNN reported on Friday, citing unnamed police sources. The discarded backpack, potentially loaded with evidence, was missed on a first pass but found on a subsequent, expanded search through the 843-acre (341-hectare) park, CNN said.

Tisch said in an interview with CNN on Friday that authorities now believe he has left New York City, after new video emerged showing him arriving at the city's main bus terminal. 

New York Police Department Chief of Detectives Joseph Kenny, in the same interview, said the video shows the suspect climbing into a taxi that took him to the Port Authority bus station.

"We have video of him entering the Port Authority Bus Terminal. We don't have any video of him exiting so we believe he may have gotten on a bus," Kenny said. "Those buses are interstate buses. That's why we believe he may have left New York City."

The expanded hunt comes after security experts cautioned that the first 48 hours after such a crime are the best window of opportunity to catch a gunman, a timeframe that has now passed.

"The clock is running," said Felipe Rodriguez, a former NYPD police detective and an adjunct professor at the John Jay College of Criminal Justice.

Rodriguez said solving the case is like putting together a difficult jigsaw puzzle.

"You start from edges first and work your way in, but right now they might not have all the pieces. But the case is moving forward," he said.

PHOTOS AND CLUES

Police believe the suspect arrived in New York 10 days before the shooting on a Greyhound bus that originated in Atlanta and checked into a Manhattan youth hostel using a fake ID from New Jersey, several media outlets reported. Reuters has not independently verified this account.

Police offered a $10,000 reward for information leading to an arrest and conviction.

UnitedHealth ( UNH ) is the largest U.S. health insurer, providing benefits to tens of millions of Americans, who pay more for healthcare than people in any other country. 

Thompson joined UnitedHealth ( UNH ) in 2004 and became the CEO of UnitedHealthcare, a unit of UnitedHealth Group ( UNH ), in April 2021.

Following the attack, UnitedHealth ( UNH ) and several other health insurers including CVS Health ( CVS ) and Centene ( CNC ) took down pictures of executives from their corporate websites in an apparent tightening of security measures.

Centene ( CNC ) said late on Thursday it would no longer hold an in-person investor day next week, and that the event would be streamed.

The words "deny," "defend" and "depose" were carved into shell casings found at the scene, police sources told several media outlets. A New York City Police Department spokesperson would not comment on the report.

The words evoke the title of Jay Feinman's 2010 book critical of the insurance industry "Delay Deny Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It." 

Feinman, a professor emeritus at Rutgers University Law School, declined to comment.

Detectives believe the perpetrator was experienced with firearms based on how he slowly and deliberately carried out the shooting, CNN reported, citing police sources who spoke on the condition of anonymity because the investigation was ongoing.

Security video showed the shooter, wearing a hooded sweatshirt, ski mask and a gray backpack, walking up behind Thompson, raising his handgun fitted with a silencer and firing at his back. Police said the gunman arrived outside the hotel several minutes before Thompson and waited for him to walk past before firing, ignoring other passers-by. 

Before the report about the backpack, CNN, whose reporter John Miller is a former NYPD deputy commissioner, said police found a phone in an alley that the gunman ran through and also recovered a water bottle the shooter bought just minutes before the attack. 

A fingerprint on the water bottle was too smudged to provide further clues about the shooter, the New York Times reported, citing a senior law enforcement official.