'Never underestimate the power of the public': Inside the 6-day hunt for CEO Brian Thompson's killer

Just six days after a masked gunman shot and killed UnitedHealthcare CEO Brian Thompson in a brazen, targeted attacked, police have nabbed a person of interest in the case thanks to a tip from a McDonald's employee ? the latest example of the public helping police track down a suspect in a high-profile killing.

Investigators uncovered "an enormous amount" of forensic and video evidence linked to the suspect, NYPD Chief of Detectives Joseph Kenny told reporters Monday, including bullet casings inscribed with words, a backpack possibly worn by the suspect, and a water bottle that reportedly contained DNA.

But Kenny said the linchpin in the case was likely the release of photos of the person of interest that were widely published and went viral on social media. Hundreds of tips poured in after investigators released the photos and offered a $60,000 reward for information leading to the arrest.

He was identified as Luigi Mangione, 26, and was detained in the central Pennsylvania town of Altoona on firearms charges Monday, New York City Police Commissioner Jessica Tisch said. She said this was the third recent case in which the public has helped the police track down a person of interest.

"We should never underestimate the power of the public to be our eyes and ears in these investigations," Tisch said.

Thousands of hours of video leads to hundreds of tips

In a city of nearly 9 million people, it's easy to disappear in plain sight, said law enforcement experts including David Carter, a professor of criminal justice at Michigan State University and director of the university's intelligence program.

Investigators tracked the person of interest's movements around the city primarily through surveillance cameras. Cameras captured him at a Starbucks before he approached Thompson from behind and shot him at close range.

The man then fled on foot and rode an electric bike into Central Park. He caught a cab around 7 a.m. on the Upper West Side and was seen on video at an uptown bus station about 45 minutes later, Kenny said.

After an "extensive video canvass" of thousands of hours of footage, police released the first image of the person of interest on Thursday, Kenny said. That was followed by more photos, some of which had a clearer view of his face, now believed to be Mangione's.

"It's extraordinarily difficult to find someone who doesn't want to be found," Carter said. "It takes a long time to do these types of analysis."

New York City has the one of the most advanced surveillance systems of any major U.S. city, largely built after 9/11, Felipe Rodriguez, a former NYPD detective sergeant who is now an adjunct professor at John Jay College of Criminal Justice in New York, told Reuters. There are thousands of cameras in New York, and all feeds can be monitored in real time as well as reviewed for earlier footage, aided by facial recognition software.

Though investigators were able to find multiple images of the suspect, Kenny said, police weren't able to identify the man using facial recognition, possibly because of the angle of the photos or other limitations on how the police can use the technology, The Associated Press reported.

"It's not as magical as it may seem," Carter said. "It does take labor to do it."

But the release of the photos prompted hundreds of people to call in with tips, which "led to the recovery of crucial evidence," Tisch said.

She said tips from the public have played crucial roles in apprehending suspects in a recent triple stabbing in Manhattan and a gunpoint robbery in Queens, during which an NYPD officer was shot.

It's incredibly common for the public to help police identify suspects in all kinds of crimes, which is why the relationship between the police and the community is so important, said Joe Giacalone, a retired NYPD sergeant and adjunct professor at John Jay College of Criminal Justice.

As the UnitedHealthcare executive's killing unleashed a wave of vitriol toward the health insurance industry, the FBI joined the search as it stretched into its fourth day and the reward money was increased by $50,000, which likely added to the public's incentive to stay vigilant, Giacalone said.

"The whole country was looking for this guy," Giacalone said. "He had no friends."

Search involved 'a combination of old-school detective work and new-age technology'

Despite speculation that the shooter may have been a professional, the killer left behind several key clues and investigators worked quickly to process forensic evidence, including DNA, fingerprints and IP addresses, Tisch said.

Police were able to recover DNA from a water bottle they believe the shooter bought at a Starbucks close to the hotel, the New York Post and The New York Times reported Friday. But to identify the gunman with DNA, investigators would need to find a match in a law enforcement database like CODIS, the Combined DNA Index System. Police also could have tried to use commercial databases to identify the suspect through genetic genealogy, but that would have taken even longer, Giacalone said.

Investigators also found a cellphone outside the hotel and began to analyze it forensically, multiple news outlets reported last week, a process that can be difficult depending on the brand of the phone and whether it is locked, Carter said.

Police can collect a great deal of information from a person's cellphone without physically unlocking it, particularly if the user backs up their phone’s data to the internet. But in some cases, text messages, photos and communication on encrypted apps are accessible only directly from the phone.

Outside firms have had to help authorities unlock phones after Apple and Google rebuffed law enforcement’s attempts to force them to unlock phones connected to criminal investigations.

Officers also interviewed potential witnesses, and the department deployed drones, canine units, scuba divers and planes to search the area, Tisch said.

"This combination of old-school detective work and new-age technology is what led to this result today," Tisch said.

Learning his name

Kenny said law enforcement did not know the person of interest's name before Mangione's arrest, but police may have had more information about him than they shared with the public during the search, said Lenny DePaul, former chief inspector and commander of the U.S. Marshals New York/New Jersey Regional Fugitive Task Force.

Days before the arrest, New York City Mayor Eric Adams Adams declined to tell reporters whether investigators had the man's name, according to the New York Post.

“We don’t want to release that now,” Adams said Saturday. “If you do, you are basically giving a tip to the person we are seeking, and we do not want to give him an upper hand at all. Let him continue to believe he can hide behind the mask."

What's next in the investigation?

Kenny said the case is "still active and ongoing" and investigators are working to track Mangione's path from New York to Altoona. He said NYPD detectives were going to Pennsylvania to interview him.

At this point, Giacalone said, the main focus will be determining a motive as investigators pore through cellphone records and online activity.

Charges could come in the next few days as police work with prosecutors to "collect enough evidence that they can prove the case beyond a reasonable doubt," Carter said.

Contributing: John Bacon, USA TODAY; Reuters; Andrew J. Goudsward, Asbury Park Press

This article originally appeared on USA TODAY: Who killed health care CEO Brian Thompson? How the search unfolded