An unknown gunman approached UnitedHealthcare CEO Brian Thompson from behind and opened fire outside New York’s Hilton Midtown in Manhattan on Wednesday morning. He then disappeared into the streets of the Big Apple, sparking a large-scale manhunt.

Authorities have not revealed a motive, but after shell casings were found at the scene with the words “deny,” “depose” and “defend” written on them – a possible reference to the book “Delay, Deny, Defend: Why Insurance Companies Don. “ “Not paying claims and what to do about it” – there was speculation that the killing may have been born out of resentment over a rejected claim.

According to The Star-Tribune, Thompson was separated from his wife, Pauline, at the time of his death, and the Minnesota-based company’s parent company laid off workers in several states in September.

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This 2017 file photo of Brian Thompson was published via Businesswire when he was named CEO of the UnitedHealthcare Unit in 2017. (Businesswire)

As police retrace the would-be assassin’s trail and search for a killer, here’s a timeline of how things went down.

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A screenshot from surveillance footage released by the NYPD shows a person of interest wanted in connection with the shooting of UnitedHealthcare CEO Brian Thompson, who was killed in Midtown Manhattan on Wednesday, December 4, 2024. (NYPD Crime Stoppers)

Police sources told Fox News that the gunman rode a Greyhound bus from Atlanta to New York City in the days before the attack.

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

A surveillance camera captures a person involved in the murder of UnitedHealthcare CEO Brian Thompson. (Gideon Platt/Stage Star Deli)

Wednesday December 4th

Around 6:14 a.m.: Before the attack, the NYPD recovered surveillance video from a downtown 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 several times minutes later.

The NYPD released still images of the alleged assassin of the UnitedHealthcare CEO at a Starbucks store near the crime scene shortly before it happened. (NYPD)

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

The footage shows a masked man waiting outside the venue, walking up behind Thompson, who was coming from another hotel across the street, and shooting him from behind – in the leg, through the back and through the 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 taken to a hospital and pronounced dead.

A surveillance image released by the NYPD shows the suspect in the shooting of United Healthcare CEO Brian Thompson on Wednesday, December 4, 2024. (NYPD Crimestoppers)

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Cameras show the murderer’s path leading away from the crime scene. He dropped a phone in an alley between 54th and 55th Streets, which authorities examined for fingerprints and other clues. He arrived on the Avenue of Americas, got on a bicycle and rode north toward Central Park. When he left the park, he was not wearing a backpack, which was seen in previous surveillance videos.

Around 7 a.m.: The video shows a man who appears to match the shooter’s description on a bicycle on the Upper West Side, more than 30 blocks from the crime scene and on the other side of Central Park. He is no longer wearing the backpack that was captured on earlier surveillance video.

On Friday morning, police searched the park for evidence.

The NYPD is offering a $10,000 reward in the case and asks 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 treated confidentially.

Fox News’ Alexis McAdams, Julia Bonavita and Christina Coulter contributed to this report.