The suspect, identified as Luigi Mangione, 26, was arrested in Altoona, Pennsylvania after he was spotted at a McDonald's by someone who believed he resembled the gunman, officials said at a news conference.
Mangione was found with a firearm and silencer consistent with the weapon used to shoot Thompson, New York City Police Commissioner Jessica Tisch said, as well as clothing and a mask similar to those worn by the killer.
He had multiple fraudulent identifications, including a fake New Jersey ID that matched the one used by the gunman to check into a Manhattan hostel days before the shooting.
Police also found a handwritten document that speaks to "both his motivation and his mindset," Tisch said.
Mangione has been arrested on firearms charges by Altoona police and New York detectives are on their way to Pennsylvania to question him, Tisch said.
Thompson, 50, was gunned down outside a Manhattan hotel early on Wednesday morning by a masked man who appeared to wait for his arrival before shooting the executive from behind.
Police had not publicly identified a motive but have said Thompson appeared to be deliberately targeted.
The words "deny," "defend" and "depose" were carved into shell casings found at the scene, several news outlets have reported.
The words evoke the title of a book critical of the insurance industry published in 2010 titled Delay, Deny, Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It.
The suspect ran from the scene and then rode a bike into Central Park.
Surveillance video captured him exiting the park and taking a taxi to a bus station in northern Manhattan, where police believe he used a bus to flee the city.
Thompson, a father of two, had been CEO of UnitedHealth Group's insurance unit since April 2021, part of a 20-year career with the company.
He had been in New York to attend the company's annual investor conference.
A UnitedHealth spokesperson declined to comment on the latest reports.
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