New York, New York - New York police said Friday they believe the man suspected of gunning down UnitedHealthcare CEO Brian Thompson may have fled the city on a bus following the murder.

The development deepens the mystery surrounding the fresh-faced suspect, an image of whom was released by investigators Thursday and who has now been on the run for almost three days.

The masked assailant was caught on camera entering a bus station in a northern neighborhood of Manhattan in the wake of Wednesday's slaying, but he could not be identified exiting the facility on foot, a police spokesperson confirmed to AFP.

"They believe he's not in New York City," the spokesman added.

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Dozens of bus and coach routes run to other states from the George Washington Bridge Bus Station.

The image of the smiling suspect was obtained from a youth hostel where the gunman apparently stayed before the hit, detectives said, with media reporting he lowered his mask to flirt with a receptionist.

Police Commissioner Jessica Tisch said Friday that detectives had access to tens of thousands of cameras around the city, allowing them to secure the image, a significant breakthrough in the case.

"We had to go through lots of video evidence to get that one money shot with the mask down," she told CNN.

The FBI, which said it was assisting the NYPD, also released a wanted poster with two surveillance photos of the suspect, offering a reward of up to $50,000 for information leading to his capture.

The gunman had on Wednesday sprayed Brian Thompson, CEO of UnitedHealthcare – one of the country's largest medical insurers – with bullets in front of bystanders, in an audacious attack captured by a surveillance camera and now seen by millions.

UnitedHealthcare is a major player in the lucrative American health care market, and the parent group had revenues of $100.8 billion in the third quarter of the year. Thompson's compensation package in 2023 was $10.2 million, according to a regulatory filing.

Police have yet to suggest a motive and would not confirm a New York Times report that the words "delay" and "deny" – often used by insurance companies to reject claims by millions of Americans, often leading to deaths – were written on shell casings found at the scene.

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Camera footage showed the suspect fleeing on foot, before getting on an e-bike and heading in the direction of Central Park, where what is believed to be his backpack was discovered.

Law enforcement sources said the suspected shooter traveled to New York via bus last month from Atlanta, Georgia.

NYPD chief of detectives Joseph Kenny would not confirm reports that a silencer was used on the murder weapon, saying that the question would be part of the investigation.

Officers have confirmed that a cell phone as well as DNA from a coffee cup had been recovered from the scene.