CNN
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He killed a high-profile CEO on a sidewalk in America’s largest city, where thousands of surveillance cameras monitor millions of people every day.

But the man who killed UnitedHealthcare CEO Brian Thompson outside a busy hotel keeps evading capture. Now authorities believe he may have escaped from New York – meaning the elusive shooter could be anywhere.

It could take weeks to find and search through a massive amount of video footage from all the locations the shooter may have traveled.

Police believe the suspect arrived in New York City 10 days before the killing – on November 24, a police official told CNN. Throughout his stay, the suspect appeared on camera several times – but always kept the hood over his head and wore a mask in public places.

“He knows he’s on camera — it’s New York,” said John Miller, CNN’s chief law enforcement and intelligence analyst.

Police are searching and combing through countless hours of video footage in hopes of finding more clues, such as whether the suspect met anyone in the city.

“It will take weeks. … They will capture every step of his journey on video,” Miller said. “They will make a film of his every move.”

While the shooter meticulously planned many parts of his crime and escape, he may be surprised at “how far the NYPD will go in collecting video,” said former NYPD Department Chief Kenneth Corey.

“And they’re not just going to take it from the crime scene to his escape route,” Corey said. “You’re actually going to rewind now and try to take into account all the ten days that he spent in New York City. And I don’t think he expects that.”

Just minutes after Thompson was shot Wednesday, surveillance video captured the suspect riding an electric bicycle into massive Central Park at 6:48 a.m

With an area of ​​843 hectares, Central Park is larger than the country of Monaco. “It’s a big park and complicated terrain,” Miller said.

The suspect apparently left the park within a few minutes. After reviewing security footage, police believe he may have left the park via the West 77th Street exit – but without the “distinctive gray backpack” that was seen on the suspect during the shooting.

At 7 a.m., a man resembling the suspect was spotted riding a bicycle on West 85th Street, private surveillance footage shows. Law enforcement told CNN they have reviewed the video and believe it likely shows the shooter.

We now have a photo showing the suspect’s unmasked face. Why didn’t anyone recognize him and report him to the police?

Some have portrayed the killer as a man carrying out vigilante justice against a health care system that they say values ​​profits over patients’ lives, which could dampen some people’s motivation to report possible sightings of him.

The evidence suggests that the shooter viewed himself as a “Batman-like figure” seeking justice on behalf of the people he believed he represented, said Bryanna Fox, a professor of criminology at the University of South Florida.

The words “delay” and “standoff” were written on a live cartridge and shell casing associated with the shooter, law enforcement sources told CNN. The police are investigating whether these words indicate a motive. The words are similar to a popular insurance industry phrase: “delay, deny, defend.”

Tens of thousands of social media users made fun of the health insurance boss’s death and showed little understanding for the killing. A post from UnitedHealthcare’s parent company mourning Thompson’s death received more than 82,000 reactions as of Friday; 76,000 of them were laughing emojis.

An image of the suspect with his mask down – taken during a flirtatious moment with a hostel worker – is the best photo yet to identify the suspect, authorities said.

But contrary to popular belief, facial recognition software doesn’t always link a suspect’s face and identity, said Donnie Scott, CEO of IDEMIA Group, which specializes in facial recognition technology.

“Most Americans may believe that law enforcement has images of everyone in the United States. That’s not true at all,” he said.

“If he’s not a New York resident and has never been arrested, there’s a good chance he won’t show up in their crime database or mugshot archive,” Scott said.

“What will likely happen across all law enforcement agencies is that they will look for this image in their local systems to see if this perpetrator exists in their galleries.”

Some believe that police can easily verify a suspect’s face using Department of Motor Vehicles driver’s license photos. But the reality is not that simple.

“It is a legal approval process. By law, New York State does not have access to the DMV database for law enforcement purposes,” Scott said. “It requires cooperation and information sharing, as well as a reason and willingness from the relevant authorities to be allowed to share this by law.”

Facial recognition technology should not be used solely to identify a suspect, Scott said.

“Our job is to make the work of law enforcement authorities easier. When you think about the actual basis and use of facial recognition technologies, it’s about sifting through the millions and millions of images to reduce them to a small, small subset that the expert can use for identification,” he said.

“Face is not a fingerprint. It’s not DNA. It is not used for a positive match. It is used to get to a small enough number that the expert can say, “Yes, I think we have an investigative lead here.” What other evidence might we have that could link this person to the crime? ‘“

Police are investigating a phone found in an alley where the suspect was running. However, extracting data from a phone can be extremely difficult depending on the phone model and operating system.

“This is something that law enforcement at all levels have struggled with over the last decade… and that’s because of the ubiquity of end-to-end encryption and these incredibly secure locking programs that we have on most phones,” said he former FBI deputy director Andrew McCabe.

During his time at the FBI, he recalled: “When we took an iPhone into an important case, whether we could participate depended not only on the model of phone it was, but also on the specific iOS or not.” version it ran.”

For example: “Maybe you could get into an iPhone 7 with iOS, but not an iPhone with a different iOS,” McCabe said. “It’s a very complex matrix of hardware and software, and that to some extent determines whether law enforcement is able to bypass this locking mechanism.”

Even if authorities can “open the phone,” the technical challenges may not yet be resolved, he said.

“Once you start making calls, you’re faced with the problem of encrypted content,” McCabe said. “Some users are not as disciplined and save things like text messages and similar records on their phones so you can read them on the device. But others are better at expiring and automatically deleting messages.”

Investigators recovered possible DNA traces from the cell phone that was left behind and from a water bottle that the suspect probably drank from. The potential DNA evidence has been turned over to a lab for testing, a senior law enforcement official told Miller.

However, the presence of DNA data may not be enough to identify the suspect.

If the suspect has previously committed a crime and his DNA has been entered into a law enforcement database, it may be easier to identify him. However, if the suspect previously had a clean record, it becomes more difficult to identify him using DNA.

“If they’re able to take DNA … but there’s no match to those DNA samples, it becomes very difficult,” said Callahan Walsh, co-host of “America’s Most Wanted.”

There is a possibility that the suspect could be isolated using genetic genealogy if one of his relatives enters his DNA information into a public database.

“You can bring in familial DNA, but it takes a little longer to process,” Walsh said. “There must also be a match in a database somewhere with one of his family members.”

Even though the suspect may have left New York, the NYPD will continue to look for clues, said Commissioner Jessica Tisch.

“We are currently processing an enormous amount of evidence in this case,” Tisch said on Friday.

“We already have a lot of forensic evidence, fingerprints and DNA evidence,” as well as “extensive camera footage” of the suspected shooter’s movements through the city, she said.

But despite the wealth of evidence, finding the suspect was an arduous task, McCabe said.

“It’s a lot more complicated than it seems from the outside.”

CNN’s Mark Morales, Shimon Prokupecz, Tami Luhby, Claire Duffy, Elise Hammond, Erin Burnett, Brynn Gingras and Courtney Fennell contributed to this report.