A motive wasn't immediately clear, but Gov. Greg Abbott said Dimitrios Pagourtzis, 17, wrote about planning the attack in journals on his computer and in his cellphone that police obtained. That was inconsistent with the portrait painted by his friends - a reserved young man, an athlete who had discussed wanting to own guns but who was said not to have given warning signs of impending violence.
"Not only did he want to commit the shooting but he wanted to commit suicide after the shooting," Abbott said, adding that Pagourtzis told authorities he "didn't have the courage" to take his own life.
Pagourtzis was held without bond in the Galveston County jail on charges of capital murder, said the county sheriff, Henry Trochesset. Abbott said both weapons were owned legally by the suspect's father.
It was not clear whether the father knew his son had taken them. Authorities urged parents nationwide to lock away their guns to keep them away from children.
Abbot said at a news conference that "unlike Parkland, unlike Sutherland Springs, there were not those types of warning signs." He was referring to the Feb. 14 school shooting in Florida and one in November inside a church in a town near San Antonio. Abbott said the early investigation showed no prior criminal history for Pagourtzis - no arrests and no confrontations with law enforcement.
"The red-flag warnings were either nonexistent or very imperceptible," Abbott said, though he acknowledged Pagourtzis had recently posted a picture of a T-shirt reading "Born to Kill" on his Facebook page.
His Facebook page also listed a possible interest in the United States Marines Corps, "starting in 2019." ABC News reached out to the Pentagon, and they said they do not have record of anyone with the last name Pagourtzis in Marine Corps.
Pagourtzis describes himself as an atheist for religious views, and "I hate politics" for political views. He also posted photos of himself, his gaming station setup, and t-shirts. He posted a photo on April 30, 2018 of a t-shirt that says "Born to Kill."
On the same day, he also posted a photo of a jacket with pins of various symbols (Communist party, Iron Cross, Rising Sun, baphomet, and Cthulhu).
The Santa Fe Indians Freshman Football website lists a "Dimitrios Pagourtzis" with jersey #73, position DL with graduation year of 2019.
FIRST LOOK: This is the mugshot of 17-year-old Dimitrios Pagourtzis. He is accused of killing at least 10 people at #SantaFeHighSchool; WHAT WE KNOW --> https://t.co/y0nL4mGNeZ #SantaFe #SantaFeShooting #SantaFeHighSchool pic.twitter.com/f3XfDj8Hgx— ABC13 Houston (@abc13houston) May 18, 2018
Tristen Patterson, a 16-year-old junior at Sante Fe, considered Pagourtzis a friend. He said Pagourtzis was into video games that simulated war, and that he sometimes talked about guns - firearms that he liked or wanted to get. "But he never talked about killing people or anything like that," Patterson said.
He said Pagourtzis didn't show signs of being bullied but also rarely talked about himself. In one of their classes, Pagourtzis would sometimes enter the room "acting a little bit down or sad. A little bit sluggish," Patterson said.
"But he never talked about why," he said.
The Assumption of the Virgin Mary Greek Orthodox Church in Galveston has posted photos of their "Galveston Nisiotes" - a Greek Dance Troupe performing at the Galveston Heritage Festival on its website. In one of the photos, a "Dimitri Pagourtzis" is listed in the photo caption. The suspect's mother's Facebook page also includes a similar photo of the same Greek dance troupe.
Father Stelios Sitaras of Assumption of the Virgin Mary Greek Orthodox Church in Galveston, Texas, said he met Pagourtzis when the young man danced with a group as part of an annual festival in October. He said the Pagourtzises are members of a nearby parish.
Sitaras said he had never heard of the teen being in any sort of trouble.
"He is a quiet boy," the priest said. "You would never think he would do anything like this."
Authorities say Pagourtzis was armed with a shotgun and a .38 revolver that belonged to his father. They believe those weapons were legally owned by his father.
Journals, a computer and cell phone recovered during the investigation suggested that not only did Pagourtzis intend to carry out the shooting, but he planned to commit suicide afterward, authorities say. He did not attempt to kill himself, but instead, surrendered to police.
Pagourtzis has been booked into the Galveston County Jail on a charge of capital murder. He is being held without bond.
Due to his age, if convicted, Pagourtzis would not be eligible for the death penalty.
During their investigation, authorities obtained warrants to search two homes. They found various explosive devices in a home and vehicle, as well as on the school's campus.
They are urging the community to be on the lookout for suspicious items and to report anything unusual to 911.
The Associated Press contributed to this report.