Dimitrios Pagourtzis was seen entering the low-slung building at Santa Fe High School on Friday morning, armed and wearing a trench coat.

Like others who’ve terrorized American students, the 17-year-old planned to kill his peers and then himself, the authorities said.

He opened fire, and the school erupted in chaos. An alarm clanged, and in the art room, bloodied students cried for help.

But once the bullets pierced his classmates’ bodies, the suspect surrendered, said Gov. Greg Abbott of Texas, and admitted “that he didn’t have the courage to commit the suicide.”

On Friday, as a small Texas town grappled with the deaths of 10 of their own, a sketchy profile of another violent young man came to the fore, the disturbing details of his life blurring with those of the school gunmen of the past.

“Born to Kill” appeared on a T-shirt he posted on his Facebook page, along with images of the trench coat and an explanation of its decorations.

“Hammer and Sickle=Rebellion,” he wrote. “Rising Sun=Kamikaze Tactics. Iron Cross=Bravery. Baphomet=Evil.”

Above all this, Mr. Pagourtzis posted artwork seemingly inspired by the electronic musician James Kent, professionally known as Perturbator. Mr. Kent’s music — largely instrumental — has been adopted by affiliates of neo-Nazi groups and the alt-right.

On social media Mr. Kent’s fans condemned reports linking his work to the alt-right, and Mr. Kent himself responded on Saturday.

“What happened in Santa Fe is atrocious and all my thoughts are with the victims and families,” he said in a Facebook post. “I feel it is unnecessary to further explain my position regarding this tragedy as I do not want medias to make any more assumptions about me or twist my words to fit their sensationalist agenda.”

Mr. Abbott said that the gunman had used a shotgun and .38 revolver, both of which appeared to be obtained from the suspect’s father, who legally owned them. The suspect also kept a journal, which detailed his plans for the attack and his suicide.

Dr. Chuck Burnell, the chief medical officer for Acadian Ambulance Service, said it appeared that the gunman had loaded the shotgun with buckshot and that “the high degree of lethality was because of close-range buckshot.”

By Friday afternoon, the suspect was in custody at the Galveston County jail. Federal authorities are seeking search warrants to find explosive devices at two residences.

Police said the gunman brought several of these devices into the school. It was unclear whether any went off.

Image A photo posted to the suspect’s Facebook page.

During a court appearance that lasted about two minutes on Friday evening, a Texas judge denied bond for Mr. Pagourtzis, who has been charged with capital murder and aggravated assault against a public servant. Mr. Pagourtzis, who was dressed in a jail jumpsuit and said little as he stood before a judge, requested a court-appointed lawyer. He was not asked to enter a plea.

Investigators, meanwhile, are scouring his journal, a computer and a cellphone that Mr. Abbott said showed the suspect had been planning the attack, and his own death.

On Friday, authorities intended to question two other people: One was at the scene and had “suspicious reactions,” according to the governor, and another had drawn the scrutiny of investigators.

Mr. Pagourtzis had no known confrontations with law enforcement, Mr. Abbott added. “As far as having a criminal history, he has none. His slate is pretty clean.”

In many ways, Mr. Pagourtzis was a part of the Santa Fe community.

He made the honor roll. He played defensive tackle on a school football team that was the pride of the town. His family was involved in the Greek Orthodox Church.

Valerie Martin, a teacher at the junior high school in Santa Fe, said she had Mr. Pagourtzis in her pre-A.P. language arts class last year.

She saw no signs that Mr. Pagourtzis might do such a thing, she recalled in an interview. She viewed him as bright, she said, adding he had taken part in the school’s competition for a national history contest. “He was quiet, but he wasn’t quiet in a creepy way.”

Tyler Ray, 18, a football player who said he knew the Pagourtzis family well, said that Mr. Pagourtzis showed up for summer workouts and tried hard even though he wasn’t very athletic. His family, Mr. Ray said, came to the games to support him, though they struggled to pay for equipment.

The day before the shooting, Mr. Ray and Mr. Pagourtzis went on a class field trip to Schlitterbahn, a water park in Galveston.

On Friday, though, a different young man showed up at school in Santa Fe.

In the art room, Zachary Muehe, a sophomore, was engrossed in his phone. He heard several booms and whipped around to see Mr. Pagourtzis, who was wearing the coat and the “Born to Kill” T-shirt.

Mr. Muehe said Mr. Pagourtzis had begun shooting as soon as he entered the classroom.

“It was crazy watching him shoot and then pump,” Mr. Muehe said. “I remember seeing the shrapnel from the tables, whatever he hit, I remember seeing the shrapnel go past my face.”

Mr. Muehe immediately tried to escape. He and his friends went to a back door in the classroom, which leads to a small courtyard, but the door was locked.

He went next to a ceramics closet that connects his classroom to another art classroom, and took one more look at the classroom behind him, and saw students lying on the ground.

“There was a girl on the ground,” Mr. Muehe said, “and he shot her in the head one or two times.”

“I just started running, as fast as I could to the other side of the campus,” he said. “When I got to the main part I started banging on the door and I saw one of my teachers. I say, ‘Hey, we’ve got a gun, everyone needs to get out.’”

Mr. Muehe’s mother, Christina Muehe, said that by Friday evening the entire community had been broken.

“There’s kids that are missing right now. You know they’re the ones that are probably dead. It’s gut wrenching,” Ms. Muehe said, adding she was deeply proud of her son for warning others about the danger unfolding inside the school.

