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

An unidentified gunman approached UnitedHealthcare CEO Brian Thompson from behind Wednesday morning and opened fire outside the New York Hilton Midtown in Manhattan, then vanished into the streets of the Big Apple, prompting a large-scale manhunt.

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.

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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 on before the murder, using a fake ID and paying in cash.

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.

Police released images of a person of interested wanted for questioning in case – giving a 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. 

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

The NYPD is offering a $10,000 reward in the case, asking the public to call the Crime Stoppers hotline at 1-800-577-TIPS (8477). You can also submit a tip via their website. All calls are kept confidential.  

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