UnitedHealthcare CEO Brian Thompson's 'premeditated' attack and suspect's getaway: timeline

Luigi Mangione, a 26-year-old Ivy League alumnus from Maryland, has been identified as the suspect in the Dec. 4 slaying of UnitedHealthcare CEO Brian Thompson outside the New York Hilton Midtown in Manhattan.

Authorities have not revealed a motive, but after bullet casings at the scene were found with words "deny", "depose" and "defend" written on them – a possible reference to the book "Delay, Deny, Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It" – speculation swirled that the slaying may have grown out of resentment for a denied claim.

Thompson was separated from his wife, Paulette, at the time of his death and the Minnesota-based firm's parent company laid off workers in multiple states in September, according to The Star-Tribune.

Mangione was arrested in Altoona, Pennsylvania, on Dec. 9 on charges that included showing a fake ID and unlawful possession of a firearm. He was later charged with murder in New York.

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As police retrace the suspected assassin's steps and hunt for a killer, here's a timeline of how things went down.

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In the days before the attack, police sources tell Fox News the gunman took a Greyhound bus from Atlanta, Georgia, to New York City.

He checked into a hostel near Central Park before the murder, using a fake ID and paying in cash.

Wednesday, Dec. 4

Around 6:14 a.m.: Before the attack, the NYPD recovered a surveillance video from a Midtown coffee shop showing the masked suspect, and surveillance video from outside the Stage Star Deli near the corner of 55th Street and Sixth Avenue shows him passing by multiple times minutes later.

Around 6:40 a.m: Surveillance video outside the hotel recorded the ambush.

It shows a masked man who was waiting outside the venue stepping behind Thompson, who was approaching from another hotel across the street, and shooting him from behind – in the leg through the back and through his chest.

Police arrived in less than three minutes, but the suspect had already fled. Thompson, a Minnesota native who was in New York for a work-related conference, was unconscious and unresponsive, according to the NYPD. He was rushed to a hospital and pronounced dead.

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Cameras show the killer's path heading away from the scene. He dropped a phone in an alley between 54th and 55th streets, which authorities were checking for fingerprints and other clues. He turned up the Avenue of Americas, got on a bike and headed north toward Central Park. When he emerged from the park, he wasn't wearing a backpack spotted in prior surveillance video.

Around 7 a.m.: Video shows a man who appears to match the gunman's description riding a bike on the Upper West Side, more than 30 blocks away from the crime scene and on the other side of Central Park. He is no longer wearing the backpack captured on earlier surveillance video, prompting the NYPD to spend most of Friday searching for evidence.  

By 8 a.m., the gunman is believed to have entered an uptown bus terminal and hitched a ride out of the state.

Police traced his route from the crime scene near 54th Street and Sixth Avenue up to Central Park, which he exited at 77th Street and Central Park West, Chief of Detectives Joseph Kenny told Fox News' Alexis McAdams Friday.

He walked with the bike to Columbus Avenue and then ditched it in favor of a taxi, which he took to the bus terminal. Investigators found video of him entering about an hour after the murder. He did not come out on foot, as far as investigators could tell.

Thursday, Dec. 5

Police released images of a person of interest wanted for questioning in the case – giving the public the first glimpse of an unmasked man.

It appears to have been taken at the hostel on the Upper West Side where the suspect is believed to have been staying.

Police sources told ABC News that the man was flirting with the woman who checked him into the HI New York City Hostel at West 103rd St. in the leadup to the killing. When the woman asked to see his smile, the suspect pulled down his face mask long enough to allow a camera at the building to capture a clearer image of his face. 

WATCH: UnitedHealthcare CEO shooting suspect seen on bike in surveillance video

Friday, Dec. 6

Police were searching Central Park for evidence Friday morning. Additional surveillance video taking between Midtown and the Upper West Side continued to emerge. 

Police on Friday afternoon revealed that the suspect had likely left New York and asked for the public's help bringing the interstate manhunt to a close.

Monday, Dec. 9

Authorities announce the arrest of 26-year-old Luigi Mangione, a Maryland native who obtained bachelor's and master's degrees in engineering from the University of Pennsylvania. 

He was arrested in Altoona, Pennsylvania, Monday morning after a McDonald's customer spotted him and thought he matched the suspect on a wanted poster. The customer flagged an employee who agreed and called police. He was wearing a medical mask and a beanie and working on his laptop, according to court documents.

Police responded to the location for reports of a person matching the description of the man wanted for questioning in Thompson's murder, but Altoona police said they initially took him into custody on unrelated charges.

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When officers approached, they immediately recognized him, according to court documents. However, he allegedly handed over a fake ID and gave a phony name. When police asked if he'd recently been to New York, Mangione allegedly "became quiet and started to shake."

Mangione surrendered peacefully but once he got the police station he did not speak with investigators, a law enforcement source told Fox News Digital. 

"Suspect didn’t say a word," the source said. "Refused to talk."

NYPD detectives arrived in Altoona Monday afternoon, followed by some of the department's top brass and representatives of the Manhattan District Attorney's Office.

Read the Pennsylvania arrest warrant:

He was initially arrested on charges related to forgery, the fake ID and unlawful possession of a firearm.

He was arraigned on those charges Monday evening and soon charged with murder and other charges in New York.

Read the New York arrest warrant:

The New York arrest warrant alleges that when Pennsylvania police captured Mangione, he was carrying a so-called ghost gun with a 3D-printed receiver and suppressor as well as "written admissions about the crime."

Also Monday, Thompson's family laid him to rest in a Minnesota funeral overlooked by state and local police. It was an intimate, private affair with no outsiders allowed in.

Tuesday, Dec. 10

Mangione's defense attorney tells a Pennsylvania judge his client would fight extradition, delaying his expected return to New York by more than a week.

Wednesday, Dec. 11

NYPD Commissioner Jessica Tisch says police have matched a 3D-printed gun allegedly recovered from suspected killer Luigi Mangione to shell casings found outside a New York City Hilton where UnitedHealthcare CEO Brian Thompson was gunned down from behind last week.

"First, we got the gun in question back from Pennsylvania," she told reporters at a news briefing Wednesday. "It's now at the NYPD crime lab. We were able to match that gun to the three shell casings that we found in Midtown at the scene of the homicide."

They also have his fingerprints on a snack bar wrapper and a water bottle, she said.

"We were also able at our crime lab to match the person of interest’s fingerprints with fingerprints that we found on both the water bottle and the KIND bar near the scene of the homicide in midtown," she added.

Wednesday, Dec. 18

Manhattan District Attorney Alvin Bragg announces an indictment against Mangione on more serious charges than previously expected, revealing the new first-degree murder charge includes the allegation that Thompson's assassination was an act of terror. Due to differences between New York law and most other states, prior to the terror charge, Mangione had been facing second-degree murder.

UNITEDHEALTHCARE CEO MURDER SUSPECT LUIGI MANGIONE INDICTED IN NEW YORK

"This was a killing to invoke terror," Bragg said. "This was not an ordinary killing, not to suggest that any killing is ordinary, but this was extraordinary."

NYPD Commissioner Jessica Tisch also condemned online support for the suspect.

"Luigi Mangione is now formally charged and indicted for murder, and let me be perfectly clear, in the nearly two weeks since Mr. Thompson's killing, we have seen a shocking and appalling celebration of cold-blooded murder," she told reporters. "Social media has erupted with praise for this cowardly attack."

Thursday, Dec. 19

Mangione reverses course in his fight against extradition and agrees to be returned to New York to face justice there. He showed no visible emotions as police escorted him out of the courthouse in handcuffs.

Several people outside the courthouse held signs that read "Deny, Defend, Depose," the same message that Mangione allegedly left on shell casings at the scene of Thompson's murder and words that are commonly used among insurance providers when they reject claims. 

Mangione now faces a top charge of first-degree murder, two counts of second-degree murder, seven criminal possession of a weapon charges of various degrees, and criminal possession of a forged instrument, all felonies.

Fox News' Alexis McAdams, Julia Bonavita, Greg Norman and Christina Coulter contributed to this report.