New York City police believe the man who fatally shot a UnitedHealth 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 Thomson, 50, the CEO of UnitedHealth's 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.

But Tisch said in an interview with CNN on Friday that authorities now believe he has left New York City, after new video footage emerged.

New York Police Department Chief of Detectives Joseph Kenny, in the same interview, said the footage 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," he 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 is 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. "They still haven't recovered the gun, the bicycle, the backpack. The longer it takes, they could be losing vital pieces of evidence."

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.