The masked gunman who killed the CEO of one of the largest U.S. health insurance companies in front of midtown Manhattan surveillance cameras remained free Thursday from the dragnet thrown by the nation's largest police department.

The New York City Police Department did release a new surveillance image, asking for the public's help in identifying the person in it for questioning.

The person is unmasked, in contrast to surveillance images released on Wednesday. In those images, the suspect wore a hooded jacket and a mask that concealed most of his face wouldn't 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 $10,000 US for information leading to an arrest and conviction.

Last seen riding in Central Park

UnitedHealthcare CEO Brian Thompson, 50, died in a dawn ambush Wednesday as he walked to the company's annual investor conference at a Hilton hotel in midtown Manhattan. UnitedHealthcare is the largest provider of Medicare Advantage plans in the U.S. and manages health insurance coverage for employers and state and federally funded Medicaid programs, but it's not clear at this point if Thompson's slaying was related to his professional position.

Investigators recovered several 9 mm shell casings from outside the hotel and a cellphone from the alleyway through which the shooter fled, NYPD Chief of Detectives Joseph Kenny had said earlier.

Meanwhile, multiple U.S. media outlets including The Associated Press reported that the gunman used ammunition emblazoned with the words "deny," "defend" and "depose." The words on the ammunition may have been a reference to strategies insurance companies allegedly use to try to avoid paying claims; they were also the title of a 2010 book that detailed reasons why insurance companies deny claims to Americans.

An employee at a nearby hostel confirmed that police had visited the location Thursday with questions related to the probe but declined to provide further information.

Police Commissioner Jessica Tisch said at a news conference 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, said one of the law enforcement officials who spoke with 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 was last seen riding into Central Park.

Minnesota police unaware of any threats

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 for the suspect.

The insurer's Minnetonka, Minn.-based parent company, UnitedHealth Group Inc., was holding its annual meeting with investors to update Wall Street on the company's direction and expectations for the coming year. The company ended the conference early in the wake of Thompson's death.

WATCH l More on the ambush killing of influential health insurance CEO:

U.S. health insurance CEO shot dead in suspected targeted killing

Brian Thompson, the CEO of UnitedHealthcare, a major American health insurance company, was shot dead on a sidewalk in New York City on Wednesday in what appeared to be a 'brazen targeted attack,' according to police.

Thompson, a father of two sons, had been with the company since 2004 and served as CEO for more than three years.

"Brian was a highly respected colleague and friend to all who worked with him," UnitedHealth Group said in a statement. "We are working closely with the New York Police Department and ask for your patience and understanding during this difficult time."

Thompson's wife, Paulette Thompson, told NBC News that he told her "there were some people that had been threatening him." She didn't have details but suggested the threats may have involved issues with insurance coverage.

Eric Werner, the police chief in the Minneapolis suburb where Thompson lived, said his department had not received any reports of threats against the executive.