Park Avenue ​mass shooter Shane Tamura was “completely different” a decade ago as a star high-school athlete​ — before he ​began express​ing suicidal thoughts ​in recent years and gambling to excess while working nights as a security guard at a Las Vegas casino.

​The Midtown killer’s descent into homicidal madness stunned those who knew him growing up.

“Nine years ago, he was a completely different Shane,” a 28-year-old former teammate told The Post on Tuesday — the day after Tamura shot up a Manhattan skyscraper in a failed attempt to target NFL headquarters, which he blamed for alleged brain injuries despite never playing in the league.

“Compared to what’s on the news, I don’t think any one of us, none of my teammates, sensed anything like this from Shane,” said the man, who asked not to be publicized.

“He was very devoted to football, he was a very good player, but off the field he was very like, nice, positive. He never really had conflict with anyone,” the man said.

Tamura was a standout running back at Granada Hills Charter and Golden Valley high schools in Los Angeles County, garnering numerous accolades including being named player of the game five times during his junior year in 2015.

“He was very quiet. He led by his actions more than his word. He was athletically gifted. One of the best running backs that I coached,” said Walter Roby, 60, who coached Tamura during his senior year at Granada Hills, to The Post.

“He was coachable, meaning when you can ask him to do something he would do it. He was quiet, humble. He knew his athletic ability but he wasn’t boastful about that.”

Roby said he was “heartbroken” at the news his former player had perpetrated the heinous mass shooting.

“It’s just shocking. It took me a while to come to grips with it. that was tough,” the coach said.

Granada Hills Charter issued a statement Tuesday saying Tamura withdrew from the school after attending the 2015 fall semester and has had no connection with the academy since.

“We are horrified by the violence in New York. It is heartbreaking, and we extend our deepest sympathies to the victims, their families, and affected communities,” the school wrote​.

Tamura took his own life after his shooting spree, in which he killed three office workers and an NYPD officer as he stormed 345 Park Ave. in Midtown with an AR-style rifle Monday evening, apparently targeting the NFL, Mayor Eric Adams said.

Police found a rambling three-page note stashed in Tamura’s wallet in which he blamed football for his alleged struggle with CTE — the neurodegenerative disease chronic traumatic encephalopathy — and begged for his brain to be studied after his death.

The note also included an apology to someone named Rick, who sources identified as the owner of the crucial part of the gun that Tamura used. Rick was Tamura’s supervisor at the Horseshoe Las Vegas Casino, sources said.

How the shooting unfolded

• Reports of the shooting at 345 Park Ave. start coming in around 6:28 p.m.
• Shane Tamura, 27, is seen getting out of a black BMW between 51st and 52nd streets with an M4 rifle.
• He enters the lobby and turns right, where he shoots police officer Didarul Islam, 36, dead.
• Tamura guns down a woman cowering behind a pillar in the lobby, sprays more bullets and walks toward the elevator bank — where he shoots dead a security guard crouching at his desk.
• One more man reports being shot and injured in the lobby. He was in critical but stable condition.
• The gunman allows a woman to walk out of the elevators unharmed before heading up to the 33rd floor, where building owner Rudin Properties’ offices are located, “and begins to walk the floor, firing as he traveled.”
• One woman is shot and killed on that floor before Tamura shoots himself in the chest.
• It’s unclear how long the mayhem lasted. Police Commissioner Jessica Tisch posted on X at 7:52 p.m.: “The scene has been contained and the lone shooter has been neutralized.”

Detectives have located the gun’s owner and are currently questioning him, as well as the seller of the weapon’s lower receiver, which an NYPD spokesman described as “what makes the rifle fire.”

Investigators are still working to determine Tamura’s exact motive, but sources said he is believed to have suffered at least two mental health incidents in 2022 and 2024.

He received his gun permit in 2022, and a short time later notified police he was having suicidal ideations — which are not grounds for having a permit revoked, a source told The Post.

In 2024, Tamura had a second mental health contact.

Both mental-health struggled ended up with him being held for 72 hours of observation, sources said.

He also had two arrests on his record, including a 2012 trespassing charge in California and a charge in Nevada in 2023 for throwing a substance at a vehicle with intent to cause great bodily harm.

In 2021, Tamura had a pair of high-value cash transactions at Nevada casinos, sources said, including spending $16,615 at the Coast Hotel and Casino and months later winning $12,860 at a casino affiliated with Hotspur Resorts Nevada Inc.

Sources told The Post police found the prescription medication Sumatriptan, used for relieving migraine symptoms, in Tamura’s BMW, which he had double-parked outside 345 Park Ave. before waltzing into the building with his assault rifle.

The gunman’s former high school teammate told The Post he never saw any indication Tamura was struggling with mental illness.

He conceded, however, that people sometimes change over time.

“You might know someone 9, 10, 12 years ago in general life and time can change a lot of things. I don’t know what happened. It could have been some sort of mental illness, some kind of stress, something could have happened, I really have no idea.”

As for whether Tamura ever got banged up enough on the field to wind up with a CTE diagnosis — as has afflicted a number of high-profile NFL players — Roby said he couldn’t recall anything but an ankle injury that sidelined him for a single game.

Asked whether he was good enough to have a chance to get drafted into the NFL one day, Roby said it was tough to say.

“Did he have the ability to play at the college level? Definitely had that athletic ability. As far as the NFL, I don’t know. Being in the NFL, you’re talking about the 1% of the 1%. I don’t know if he had that type of skill set.”

Additional reporting by Tina Moore