'Seemed unimaginable': Luigi Mangione's former roommate reacts to arrest

The above video was taken during a Dec. 9 news conference concerning the arrest of a person of interest in the killing of UnitedHealthcare CEO Brian Thompson.

MIDTOWN, Manhattan (WPIX) – A former roommate of Luigi Mangione, the suspect in UnitedHealthcare CEO Brian Thompson's fatal shooting, told Nexstar's WPIX that he was shocked to learn of Mangione's arrest in connection with the Dec. 4 killing of UnitedHealthcare CEO Brian Thompson.

“From the person I lived with, it seemed unimaginable,” RJ Martin said of Mangione. “He was never aggressive. I never ever remember him once being upset. One of the sharpest minds I’ve ever been able to interact with.”

Martin and Mangione lived together in Hawaii, which was Mangione's last place of residence.

Mangione, 26, was taken into custody Monday after he was spotted by a customer at a McDonald's in Altoona, Pennsylvania, according to authorities. He's been charged with murder, weapon possession, forgery and other charges.

Mangione is accused of killing Thompson, 50, as the CEO was headed to an investors meeting at a hotel in New York City last Wednesday, according to authorities.

Bullet shells found at the scene had the words "deny," "defend" and "depose" engraved on them — a phrase similar to one used by insurance industry critics, according to Joseph Kenny, New York Police Department Chief of Detectives.

The NYPD spent days searching Manhattan for clues that would lead them to the suspected gunman. A backpack with fake money was found in Central Park, which is believed to be tied to Mangione.

At the time of his arrest, Mangione had a fake ID that was believed to have been used at a Manhattan hostel where he stayed before the shooting. He also had a three-page document on him suggesting he had "ill will toward corporate America," Kenny said.

Mangione is originally from Baltimore, Maryland, and graduated from the University of Pennsylvania.

His family released a statement after his arrest, saying, “Our family is shocked and devastated by Luigi’s arrest. We offer our prayers to the family of Brian Thompson and we ask people to pray for all involved. We are devastated by this news.”