New York police said Friday they believe the man suspected of gunning down a top health insurance executive may have fled the city on a bus following the brazen daylight 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 spokesman confirmed to AFP.

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

Dozens of bus and coach routes run to other US states from the George Washington Bridge Bus Station, according to the operator's website.

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 "money shot" 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 gunman 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.

Thompson was attending an investor conference in the Midtown business district.

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 -- were written on shell casings found at the scene.

Video footage shows Thompson on the sidewalk outside the New York Hilton Midtown when a man in a hooded top, and with his lower face covered, approaches from behind, then fires several shots at his 50-year-old victim, who crumples to the ground.