Words written on US CEO killing shell casings: source

A masked gunman who stalked and killed the leader of one of the largest US health insurance companies outside a Manhattan hotel used ammunition emblazoned with the words "deny," "defend" and "depose," a law enforcement official says.

The official was not authorised to publicly discuss details of the ongoing investigation and spoke to the Associated Press on the condition of anonymity.

UnitedHealthcare CEO Brian Thompson, 50, died in a dawn ambush on Wednesday as he walked to the company's annual investor conference at a Hilton hotel in Midtown, blocks from tourist draws like Radio City Music Hall and the Museum of Modern Art.

The reason behind the killing remained unknown.

The words on the ammunition may have been a reference to strategies insurance companies allegedly use to try to avoid paying claims.

Investigators recovered several 9mm shell casings from outside the hotel and a mobile phone from the alleyway through which the shooter fled, NYPD chief of detectives Joseph Kenny had said earlier.

The killing and the shooter's movements in the minutes before and afterward were captured on some of the multitudes of security cameras present in that part of the city.

The hunt for the shooter brought New York City police on Thursday morning to at least two hostels on Manhattan's Upper West Side, based on a tip that the suspected shooter may have stayed at one of the residences, according to a second law enforcement official briefed on the investigation, who requested anonymity because they were not authorised to discuss the ongoing search.

Two detectives arrived at the Kama Central Park hostel with a photo of the shooter and asked staff if they recognised the man, an employee confirmed to the AP.

They did not, the employee said, and the detectives left.

An employee at the nearby HI New York City hostel also confirmed that police had visited the location on Thursday, but declined to provide further information.

Also on Thursday, police released photos of a person they said was wanted for question in connection with Thompson's shooting.

Police Commissioner Jessica Tisch said at a news conference on Wednesday that while investigators had not yet established a motive, the shooting was no random act of violence.

"Many people passed the suspect but he appeared to wait for his intended target," she said.

Investigators believe, judging from surveillance video and evidence collected from the scene, that the shooter had at least some prior firearms training and experience with guns and the weapon was equipped with a silencer, according to one of the law enforcement officials who spoke with the AP.

Investigators are also looking into whether the suspect had pre-positioned a bike as part of an escape plan, the official said.

The shooter fled on a bike and he was last seen riding into Central Park.

Authorities were running DNA and fingerprint analysis on items found nearby, including a water bottle, that they believe the suspect may have discarded, the official said.

Security camera video showed the killer approach Thompson from behind, level his pistol and fire several shots, barely pausing to clear a gun jam while the health executive tumbled to the pavement.

Other cameras captured the initial stages of the gunman's escape.

He fled the block across a pedestrian plaza, then escaped on the bicycle.

Police used drones, helicopters and dogs in an intensive search but the killer's whereabouts remained unknown.

Police issued several surveillance images of the man, who wore a hooded jacket and a mask that concealed most of his face and would not have attracted attention on a frigid day.

Some of the photos were taken at a Starbucks coffee shop shortly before the shooting.

Police offered a reward of up to $US10,000 ($A15,500) for information leading to an arrest and conviction.