A former Southern California football coach who once coached the suspect in the deadly mass shooting inside a New York City office building is speaking out.

Good Morning America sat down with Walter Roby, the former assistant football coach at Granada Hills High School in Los Angeles, who once coached Shane Tamura during his senior year.

“It was heartbreaking, shock, still in disbelief. I did not witness any injuries — head trauma or anything like that,” Roby said. “The only injury I knew about is he sustained an ankle injury. I can’t recall if it was his left ankle, his right ankle. But that’s the only thing I was aware of or can recall is that he sustained an ankle injury.”

Roby was Tamura’s coach in 2015.

Tamura left behind several notes in which he blamed the NFL for a traumatic brain injury. Tamura has no known connection to the NFL.

The deadly mass shooting at a Manhattan office building left four people dead and one injured. The incident occurred just before 6:30 p.m. Monday in a building located at 345 Park Ave. and 52nd Street, which contains the headquarters for the investment company Blackstone and the National Football League.

In an interview with ABC News, New York City Mayor Eric Adams shared new details about the attack, describing the incident as an “intentional, sick, twisted act of violence.”

Security camera footage showed the 27-year-old Tamura’s methodical approach, Adams said.

“When you look at the tapes… there was just a total disregard for human lives,” Adams said.

The shooter allowed a woman to walk past him unharmed, while he had attacked everyone else he encountered. Among those killed were Wesley LePatner, a Blackstone employee and mother of two; Didarul Islam, an off-duty NYPD officer; security officer Aland Etienne; and Julia Hyman, a Rudin Management employee. An NFL employee was also injured in the attack.

On Wednesday morning, Deputy Mayor Kaz Daughtry recounted the heroic actions of the NFL employee who called his coworkers after he was shot in the lobby, the maid who somehow avoided being shot, and the tragic circumstances that led to the death of Hyman, a young worker on the 33rd floor.

After the 41-year-old NFL employee was shot in the lobby, he called coworkers upstairs, telling them what happened.

“He didn’t call 911. He called his friends to tell them upstairs what was happening. ‘You guys, I just got shot downstairs, get cover, you’ve got to fortify those doors, hit those panic rooms.’ He’s letting his team know. This guy is a hero,” Daughtry said.

Daughtry said Tamura then shot a security guard who was trying to get to the panic button under a desk to recall the elevators from coming down.

“Then the gunman, the suspect, goes to the elevator bank where a female comes off the elevator, she had an angel with her because he lets her walk right out,” Daughtry said.

Then, officials say the suspect got into the elevator and headed to the 33rd floor, the headquarters of Rudin Management. Daughtry said “extremely dramatic and horrifying” surveillance video showed the gunman immediately opened fire as he stepped off the elevator, shooting out the glass doors.

A maid pushing her cart fled the gunfire as the gunman ran and fired at her through the rows of cubicles. She eventually hid in an office.

“The cleaner, she hears the shots, he sees her, she makes a right, he makes a left, if she would have made that left, he would have taken her down,” Daughtry said. “She’s even grabbing her pal as she’s running down the hall and he’s firing at her.”

Meanwhile, another employee emerged from the bathroom safe room, apparently to see if the coast was clear.

As she opened the door, surveillance video shows the gunman reacting to hearing the noise of the door. He changed direction from pursuing the fleeing maid to pursuing the noise.

He found the woman, Rudin associate Hyman, and opened fire. She fled, and although she was shot, she managed to make it to a cubicle, where she collapsed in a chair and died, Daughtry said.

After apparently barricading himself on the 33rd floor, Tamura was found dead from what is believed to be a self-inflicted gunshot wound.

NYPD detectives searched the shooter’s studio apartment in Las Vegas, where they found what law enforcement sources described to ABC News as a suicide note that said, “When I look into you and dad’s eyes, all I see is disappointment. I love you, Mama. I’m sorry.”

The search also turned up a tripod for the rifle, a single rifle round, and 100 9-millimeter ammunition.

Detectives also interviewed Tamura’s supervisor at the Horseshoe casino, who sources said was very cooperative. It was he who sold the rifle to Tamura and also sold Tamura the BMW driven across the country to carry out the attack.

The entire gun was assembled and sold to Tamura for $1,400, the sources said the supervisor told detectives.

The supervisor also told detectives he legally purchased the weapon he then sold to Tamura.

According to the sources, it is not clear if the supervisor knew what Tamura was going to do with the gun once he sold it.

Who is Shane Tamura?

As a teen, Tamura 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.

During a press conference on Monday evening, NYPD Commissioner Jessica Tisch said Tamura had a “documented mental health history.”

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.