The 27-year-old gunman, identified as Shane Tamura, played football while attending Granada Hills Charter school in Los Angeles County.
Those who knew Tamura when he was a young athlete say they never saw any signs of any problems and that all he was focused on was football.
NY gunman ID'd as 27-year-old who played high school football in SoCal
Tamura, wearing body armor and carrying an M4 assault rifle, shot and killed four people, including an off-duty New York Police Department officer who was working security in the Midtown Manhattan office building.
After apparently barricading himself on the 33rd floor, Tamura was found dead from what is believed to be a self-inflicted gunshot wound.
Who is Shane Tamura?
As a teen, Tamura reportedly played football in Santa Clarita and later at Granada Hills Charter school.
"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 said in a statement. "Because this remains an active investigation, we are deferring to the NYPD and federal authorities for all further details."
Tamura's father was a Los Angeles Police Department officer who served for 27 years and retired in 2017, the agency said.
File video shows young Tamura celebrating a win out on the gridiron for Granada Hills Charter school, where he graduated in 2016 after transferring from Golden Valley High School in Santa Clarita.
Tamura is heard in the video saying, "Definitely had to stay disciplined. The coach just kept telling us don't hold your heads down...hold your heads up high and then a good result's gonna come."
Now, his former teammates are speaking out after learning Tamura is accused of the killings.
One teammate described Tamura as a "goofball".
Another, who didn't want to be identified, said Tamura was a good guy.
"He was a great teammate. He was a great guy in general. He didn't cause any problems actually at all in the locker room or on the field," he said. "He was just a guy who really enjoyed the sport, not problematic at all so when I found out the news, I was really shocked, I really couldn't believe it."
The shooter's notes
Police investigating the deadly Midtown shooting found two notes from the gunman, sources told ABC News.
One page of the first note found in Tamura's pocket accused the NFL of concealing the dangers to players' brains from playing football in order to maximize profits, sources said.
A second page mentioned CTE and blamed football. A third page asked, "Study my brain please. I'm sorry," sources said.
CTE, or chronic traumatic encephalopathy, is a brain disease linked to repeated hits to the head, often seen in military veterans and athletes, including football players, hockey players and boxers.
Tamura has no known connection to the NFL. It remains unknown if Tamura suffered from CTE, which can't be diagnosed in a living person with certainty, but doctors may suspect it based on symptoms and a history of head trauma. Tamura's brain will be examined as part of his autopsy, the New York City Office of the Chief Medical Examiner said.
The second note was found in Las Vegas, where Tamura lived, the sources said, describing it, at least in part, as an apology to his parents.
During a press conference on Monday evening, NYPD Commissioner Jessica Tisch said Tamura had a "documented mental health history." He recently drove from Las Vegas to New York, and officers found additional ammunition, a revolver, a backpack, and medication inside his vehicle.
The initial investigation shows his vehicle traveled through Colorado on July 26, then Nebraska and Iowa on July 27, and then in Columbia, New Jersey, as recently as 4:24 p.m. on Monday, with his vehicle entering New York City shortly thereafter, Tisch said.
Tamura recently lived in Las Vegas, where he was issued a concealed firearms permit by the Las Vegas Police Department. He was previously issued a work card by the Nevada Private Investigators Licensing Board, which has since expired, according to records reviewed by ABC News. His last known address was in a gated community in Las Vegas.
The motive for the deadly shooting is currently under investigation, FBI Deputy Director Dan Bongino said earlier on Monday.
ABC News contributed to this report.