GUNMAN Shane Tamura had a bright future as a star high school football player before he reportedly targeted the NFL in a sick shooting that claimed four lives.

The 27-year-old claimed his years playing the sport left him with a debilitating brain condition, and accused the league of hiding the truth behind the risks.

On Monday just before 6 pm, Tamura drove from his home in Las Vegas, Nevada, to the Midtown Manhattan skyscraper that houses the NFL and several other top-tier companies.

As the last crowd of 9-5 workers filtered out, the deranged gunman barged through the doors carrying a rifle and opened fire on security staff and bystanders.

He murdered a New York Police Department officer, took aim at a mom as she dove behind a pillar for safety, and killed a security guard before heading to the elevators.

According to New York City Mayor Eric Adams, he was seeking out the NFL office, but accidentally took the wrong elevator to the 33rd floor, where the property management firm Rubin is headquartered.

When he got out, the heartless gunman shot and killed a 27-year-old associate before turning the weapon toward his chest and firing.

In a three-page note found near his corpse, Tamura spoke extensively about football and appeared to suggest he was suffering from chronic traumatic encephalopathy (CTE).

This is a degenerative disease caused by brain trauma, and it’s been found in some former NFL players after they died by suicide.

In the note, Tamura made a desperate demand for investigators to examine his brain and determine whether he had the condition.

Years before committing the shocking crime, Tamura had a bright but ill-fated career as a high school football player.

Years-old pictures and videos show him looking hopeful and healthy as he helped to lead his teammates to victory.

In one video interview taken by the Los Angeles Daily News in 2015, Tamura was described as a “stand-out running back” and was asked to relay how he scored the game-winning touchdown.

At the time, Tamura was playing at Granada Hills High School in Los Angeles, California.

In the clip, the interview talked about how the team had to “stay disciplined” despite being down by ten points.

“We just had to keep playing, keep fighting through it, and then just hold our head up high,” he said, describing the importance of their perseverance.

The player insisted that the entire game, he told himself, “A good result’s gonna come.”

TEAMMATES SPEAK

Online statistics for Tamura show that he was ranked 146th in the nation in 2016 for kickoff return yards average.

Caleb Clarke, one of Tamura’s former teammates, described the gunman as a positive person who showed great promise.

“You never would have thought violence was something you’d associated with him,” he told NBC News.

“Everything he said was a joke.”

Clarke said the two lost touch over the years, but they had reconnected recently on social media, and he learned that Tamura was working as a security guard at a casino.

Tamura’s high school coach Walter Roby said that the player was incredibly talented and was a reliable presence.

“He came in, worked hard, kept his nose down,” said the coach.

“He was a quiet kid, well-mannered, very coachable. Whatever needed to be done, he would do.”

Clarke said that when they were in high school, many teammates talked about playing ball in college or professionally.

Despite making some impressive conference achievements, Tamura never signaled any interest in continuing with the sport.

“The only thing I can really think of is there was a point where it looked like the sky was the limit, and then it wasn’t anymore,” said Clarke.