UHC CEO Murder Person of Interest: Voice Heard in Valedictorian Speech, on Video

Armchair social media detectives have dug up a 2016 high school speech by suspected UnitedHealthcare CEO murder suspect Luigi Mangione -- and the Ivy League grad looks like anything but a cold-blooded killer.

Check out the footage ... Decked out in a snazzy suit and tie, Mangione delivered his calm and collective speech before his fellow students at the Gilman School -- an expensive private boys' school in Maryland.

One thing that jumps right out at you is just how well-spoken and proper Mangione was in 2016 when he gave this 8-minute address harkening back on their academic successes over the years. At the end, Mangione received an enthusiastic round of applause.

After he graduated from Gilman ... Mangione attended the University of Pennsylvania, an Ivy League college, where he received bachelor's and master's degrees.

But things took a turn for the worse Monday when Mangione was named as a "strong" person of interest in the slaying of UnitedHealthcare CEO Brian Thompson in Manhattan last week.

Pennsylvania police arrested Mangione after a McDonald's employee spotted him having a meal in a McD's in the city of Altoona. The worker had apparently seen widely circulated wanted posters of Mangione and put two and two together before calling police.

Cops searched Mangione and recovered his fake ID and the purported gun he used in the fatal shooting ... as well as a handwritten 2-page document that was "critical" of the healthcare industry. Mangione is currently being held in custody on an unrelated gun charge.