Pagourtzis was arrested, and authorities say he is a student at the high school. According to a probable cause affidavit, he told authorities that "he did not shoot students he did like so he could have his story told."
Two sources briefed on the investigation told ABC News that Pagourtzis studied previous mass shootings while planning his attack on Friday.
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
TEAMMATE: "He was very quiet and kept to himself"
Dimitrios Pagourtzis was known for being heavily involved in his church, St. John the Theologian in Webster.
"It is my understanding that the entire family is heavily involved in the church," said Nicholas Poehl, one of Pagourtzis' defense attorneys.
In fact, Pagourtzis is a member of Asperia Dance Troupe, part of the church's outreach program. He performed most recently at the Clear Lake Greek Festival, held every May as a fundraiser for the church.
The church has taken their Facebook page and website down. In addition, they have scrubbed all mention of the dance troupe from the website.
However, in Google archives, you can see Pagourtzis as a member of the dance troupe. Performance videos are also posted on YouTube and other social media sites.
"I know he was there Friday, Saturday and Sunday, and I didn't think anything of it at the time," said Chris Sokols, a fellow Greek who runs a Greek deli near the church.
Sokols say the Greek Orthodox community is small, and the Greek Festival was a chance to raise funds for the church.
"Having the Greek Festival, being around so many people, it's scary it could have happened there, could have affected my family personally," said Sokols.
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.