By Julio-Cesar Chavez and Rich McKay

ALTOONA, Pennsylvania (Reuters) - Investigators on Tuesday were mapping the movements of a 26-year-old man charged with gunning down UnitedHealth executive Brian Thompson on the streets of Manhattan last week, after a sprawling five-day manhunt concluded with his arrest at a fast food restaurant in Pennsylvania.

Authorities have said they are also examining whether the suspect, Luigi Mangione, was helped by an accomplice either before or after the brazen shooting, which offered few immediate clues to the gunman's identity.

Mangione was spotted at a McDonald's in Altoona, Pennsylvania, on Monday by an employee who thought he looked like the gunman in surveillance images released by police. A gun, clothing and fake identifications found in his possession all closely match those used by the shooter, police said.

He faces gun and forgery charges in Pennsylvania and was arraigned in Altoona on Monday. Later that evening, prosecutors in New York filed murder and gun charges and are expected to seek Mangione's extradition in the coming days.

While the gunman's motive remains unclear, police have said Thompson, the CEO of one of the nation's largest health insurers, was deliberately targeted.

Mangione suffered from chronic back pain that limited his daily life, according to several news accounts. His profile on X shows a background image of an x-ray with what appears to be screws and plates inserted in a lower back.

From January through June 2022, Mangione lived at the Surfbreak co-living community, similar to an adult dormitory, where he led a book club and surfed, hiked and rock-climbed, according to the Hawaiian online news site Civil Beat and other media.

The founder of the group, R.J. Martin, said Mangione's pain lingered for years, caused by misaligned vertebrae that would pinch Mangione's spinal cord, and he left for the mainland at some point for surgery.

But he went "radio silent" in June or July, Martin told Civil Beat.

At one point, Mangione suggested the group's book club read the manifesto of Ted Kaczynski - the Unabomber - as a joke, Martin said.

GHOST GUN

Mangione, an Ivy League graduate who was also the valedictorian of a private all-boys school in Maryland, had a loaded ghost gun - a firearm assembled from parts, making it untraceable - and a silencer, officials said on Monday. Both the weapon and his clothing closely resembled those used by the gunman.

He also had a large sum of cash and multiple fake identifications, including a fraudulent New Jersey ID that matched the one used by the gunman to check into a Manhattan hostel days before the shooting, according to authorities.

Police found a handwritten document that offers insight into his motivation, officials said. The writing included the phrase, "These parasites had it coming," the New York Times reported, citing a law enforcement official.

Mangione's family released a statement saying they knew only what had been reported in the media.

"Our family is shocked and devastated by Luigi's arrest," the family said in a statement posted to the X account of Maryland lawmaker Nino Mangione. "We offer our prayers to the family of Brian Thompson and we ask people to pray for all involved."

BRAZEN ESCAPE

The gunman managed to elude capture for days after the attack last Wednesday outside the Hilton hotel in midtown Manhattan.

After lying in wait for Thompson, the masked suspect shot him in the back before fleeing on foot, riding a bicycle into Central Park and eventually making his way to a bus station in northern Manhattan, where police believe he boarded a bus and left the city.

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 2010 book critical of the insurance industry, "Delay, Deny, Defend: Why Insurance Companies Don't Pay Claims and What You Can Do About It."

Thompson's murder unleashed a wave of frustration from Americans struggling to afford medical care and those who have been denied claims or care.

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.

(Reporting by Joseph Ax; Editing by Bill Berkrot)