• A gunman fatally shot UnitedHealthcare CEO Brian Thompson on December 4 in New York City.
• The NYPD launched a manhunt but has so far failed to capture the suspect.
• The suspect's evasion has revealed holes in surveillance, but police say it's just a matter of time.

The Manhattan shooting Wednesday of UnitedHealthcare CEO Brian Thompson was captured by a surveillance camera and shared on social media, where the footage racked up hundreds of thousands of views. Authorities later released full-color surveillance images of the gunman, including one that showed him with his mask pulled down and smiling.

Yet, the gunman is still at large after four days despite a citywide manhunt led by the largest metropolitan police force in the country. On Friday, authorities told CNN they believed the gunman had managed to leave the city by bus.

The gunman's ability to evade capture so far has highlighted the limits of surveillance, even in a city like New York, where authorities have access to thousands of cameras that can track millions of people daily.

"You have got to remember, he was running around a city of 9 million people," Joseph Giacolone, a former NYPD Sergeant and professor at the John Jay School of Criminal Justice, told Business Insider. "You know, it's not that easy to pick somebody up the street, especially if they're all buttoned up."

New York City is under constant surveillance by police and residents

After the September 11 terrorist attacks in 2001, George W. Bush's administration expanded surveillance across the country.

Bush signed the Aviation and Transportation Act that November, creating the Transportation Security Administration. That same year, the Bush administration ushered in the USA Patriot Act, which expanded law enforcement's ability to use surveillance.

The US Congress later created the Department of Homeland Security. The department launched a nationwide campaign — "If You See Something, Say Something" — encouraging citizens to report suspicious activity to law enforcement to prevent terrorism and other criminal acts.

Since then, the emphasis on surveillance and security has spread nationwide, including in New York City, where cameras are now everywhere.

Amnesty International, a human rights organization, said there were over 25,500 surveillance cameras in New York City in a 2022 report. The NYPD has used images from the ubiquitous cameras to track crimes and for use in facial recognition software.

The NYPD's "Facial Identification Section" received 9,850 requests for comparison and returned 2,510 possible matches in 2019 — a roughly 25% match rate. The agency said it's unaware of cases in NYC in which a person was falsely arrested due to a facial recognition match.

This May, New York City Mayor Eric Adams launched a pilot program focused on using technology to increase public safety. The "community-based security camera integration platform" will allow businesses to "voluntarily share information in real-time with the NYPD through existing closed-circuit television cameras," according to a press release.

The emergence of Amazon's Ring cameras and smartphones has added another layer of monitoring. In 2022, the NYPD said it would join and monitor the Neighbors app, where residents share information on crime and safety.

"While the NYPD will not monitor the app around the clock, it will have the capacity to view, post and respond to crime- and safety-related information posted publicly by the users of the app," a press release said.

The NYPD is "processing a tremendous amount of evidence"

Commissioner Jessica Tisch told CNN on Friday that the department had already collected "lots of forensic evidence" and was "processing a tremendous amount of evidence in this case."

She said there is also a "massive camera canvass" of the suspect's movements through the city.

Additionally, a law enforcement official told CNN that investigators found a backpack in Central Park they believe belonged to the suspect but had not officially confirmed where it came from. Authorities took the backpack for tests.

Giacolone told BI that while the shooting suspect has evaded capture for now, it will be difficult for him to elude authorities as they collect more evidence. The NYPD will be looking for what he called "the three horsemen of forensics" to solve the case, which are video surveillance, cellphone records, and internet records.

"I've been on these investigations," Giacolone said. "They know what hole he crawled out of, what hole he went back into. As far as I'm concerned, they already know who he is. They just got to find him."