Pagourtzis was arrested, and authorities said he is a student at the high school.
His Facebook page also listed a possible interest in the U.S. Marines Corps "starting in 2019."
Pagourtzis describes himself as an atheist and "I hate politics" for his political views. He also posted photos of himself, his gaming station setup, and T-shirts, with one clothing item containing the phrase "Born to Kill" posted on April 30, 2018.
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 number 73, position DL, with an expected graduation year of 2019.
TEAMMATE: "He was very quiet and kept to himself"
The Assumption of the Virgin Mary Greek Orthodox Church in Galveston 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.
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 cellphone recovered during the investigation suggested that not only did Pagourtzis intend to carry out the shooting, but he planned to commit suicide afterward, authorities said. He did not attempt to kill himself, but instead, surrendered to police.
Pagourtzis was 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.