(CNN) The teenager accused of carrying out the mass shooting at Santa Fe High School on Friday was known as a quiet student who often wore a trench coat to school.

Dimitrios Pagourtzis, 17, is being held on charges of capital murder and aggravated assault of a public servant in relation to the school shooting, Texas authorities said.

Pagourtzis allegedly used a shotgun and a .38-caliber revolver to kill 10 people -- eight students and two teachers -- and injure 13 others, and he confessed that he acted alone in the shooting, according to a probable cause affidavit.

Here's what we know so far about the 17-year-old suspect:

A quiet student

Dimitrios Pagourtzis (pronounced: di-MI-tree-oas pag-OR-cheez) began attending Santa Fe High School in August 2015, according to his Facebook page.

Pagourtzis played on the 2016 JV football team and appeared on the 2015-2016 boys freshman football team. He was listed on the Santa Fe Junior High sixth-grade honor roll in 2012.

"I've talked to him once or twice. I had my advisory class, which is after third period, with him," Santa Fe High student Mateo Twilley said. "He was really quiet, and he wore, like, a trench coat almost every day."

This image of a trench coat was taken from a Facebook page belonging to Dimitrios Pagourtzis.

Student Aidan Gomez noted that Pagourtzis always wore a trench coat.

"He was kind of a quiet kid," Gomez said. "Every time you'd try to start a conversation with him he'd just kind of like laugh, wouldn't really continue on with the conversation. ... He didn't like interacting with other students."

Gomez said he was surprised when he heard Pagourtzis was suspected of being the shooter.

"I didn't think he was that kind of kid," Gomez said.

'Never seen a problem'

Nicholas Poehl, one of Pagourtzis' attorneys, said that his client did not appear to have a history of mental health or legal issues.

"At this point, from everything we know, it doesn't appear like this kid had the kind of warning signs of someone that you would normally associate with someone involved in something like this," he said.

Pagourtzis lived with his parents and had not previously been employed, according to court documents.

Neighbors in nearby Alvin, Texas, say Pagourtzis grew up there on a tree-lined street.

"Never seen a problem out of that child," said Richard Wallace, recalling how Pagourtzis would come around every year for Halloween.

Another neighbor, Bobbie Owen, said Pagourtzis and his younger sister helped her carry in Christmas packages last year.

Bertha Bland, 54, said her grandson Stephen Bland had been friends with Pagourtzis since the fourth grade. The two would eat lunch together and play video games after school, up until sometime last year when Pagourtzis pushed Stephen away for new friends for unknown reasons, she said.

Bland said her grandson and Pagourtzis both had an interest in guns and violent video games. Pagourtzis' mother told Bland her son would stay up all night playing video games, according to Bland.

Bland also said Pagourtzis came from what seemed to her like a strict household, where she said he barely had a social life.

Stephen Bland was questioned for about an hour by investigators Friday night, his grandmother said. He told investigators Pagourtzis confided in him about keeping a journal with "thoughts and stuff."

Talking to police

According to a probable cause affidavit signed by the Galveston County Sheriff's Office, Pagourtzis waived his Miranda rights during an interview at the Santa Fe Police Department.

Pagourtzis told officers he dressed in a trench coat, carried a Remington 870 shotgun and a .38 caliber pistol and shot multiple people "with the intent of killing people," the affidavit said.

The affidavit said Pagourtzis didn't shoot students he liked "so he could have his story told."

Pagourtzis made a brief initial appearance in court Friday night, standing with his wrists bound, answering questions, "Yes, sir" and "No, sir," and looking down at the floor.

He did not enter a plea and bond was denied. Pagourtzis told the judge he is a citizen of the United States and requested a court-appointed attorney.

Searching for a motive

Pagourtzis is not known to have any criminal record, Gov. Greg Abbott told reporters.

Officials have not released any information about motive.

Authorities found writings in the suspect's journals that indicated he wanted to take his own life, Abbott said.

There was information "contained in journals on his computer and his cellphone that he said that not only did he want to commit the shooting, but he wanted to commit suicide after the shooting," Abbott said.

"As you probably know, he gave himself up and admitted at the time he didn't have the courage to commit the suicide, that he wanted to take his own life earlier," Abbott said.

One of the victims, Shana Fisher, had rejected the suspect's advances for months , her mother Sadie Baze said. About a week ago, she stood up to him in class to proclaim she would not go out with him, Baze said.

Baze said she believes the incident embarrassed the suspect so much that he targeted Shana. "One of the shotgun shells was for my daughter," she said.

His social media footprint

This image of a T-shirt was taken from a Facebook page belonging to Dimitrios Pagourtzis.

Pagourtzis has a social media footprint that included an image of a custom T-shirt emblazoned with the words, "BORN TO KILL" posted on Facebook and several images of a black duster jacket with Nazi, communist, fascist and religious symbols.

In his Facebook bio he showed interest in joining the US Marine Corps, claiming to be "starting in 2019."

The military said it had no record of Pagourtzis filling out paperwork at a recruiting station.

The weapons

Abbott told reporters the alleged shooter used a shotgun and a revolver that were legally owned by his father.

The shooter was hiding a shotgun underneath a long coat Friday morning when he walked onto the campus, Lt. Gov. Dan Patrick said.

Galveston County Judge Mark Henry told reporters Pagourtzis attempted to use explosive devices but none were functional.

"There were CO2 (carbon dioxide) canisters wrapped up with duct tape, but no way to detonate, and a pressure cooker with an alarm clock, and some nails but no explosive device, so you've got to treat them like they are potentially lethal and go from there," Henry said.

The suspect's family

The Pagourtzis family issued a statement on Saturday extending prayers and condolences to the victims and thanks to first responders.

"We are as shocked and confused as anyone else by these events that occurred," the statement said. "We are gratified by the public comments made by other Santa Fe High School students that show Dimitri as we know him: a smart, quiet, sweet boy. While we remain mostly in the dark about the specifics of yesterday's tragedy, what we have learned from media reports seems incompatible with the boy we love.

"We share the public's hunger for answers as to why this happened, and will await the outcome of the investigation before speaking about these events. We have been and will continue to cooperate with the authorities conducting the investigation, and ask for the public's patience while it moves forward.

"We ask the public to please extend privacy, both to the victims and to our own family, as all of us try process these events, and begin the healing process."

The investigation

Investigators believe Pagourtzis acted alone, a law enforcement official told CNN.

Earlier, Abbott and other officials indicated that two other people were being interviewed to see whether they were involved. But authorities now believe those two were not connected to the crime, the official said.

Investigators were searching a trailer nearby where it is believed explosive devices were assembled, according to a law enforcement source, who says a pressure cooker was found.

Explosive devices also were found in surrounding areas. Speaking at a news conference, Abbott said the suspect's vehicle will be searched once a warrant is obtained.

Suspect's attorneys: 'We've got a lot of work to do'

Attorneys Nicholas Poehl and Robert Barfield said they are representing Pagourtzis. They said in a recorded interview Friday that they had met with their client and had been connected to him through a "friend of a friend."

"This is a very young kid. He's in very trying circumstances," Poehl said. "To say he's doing well surely wouldn't be accurate right now."

Of the Pagourtzis family, Poehl said "it's been a very tough day for them."

"I think every parent probably instinctively knows they don't know everything about their kids, but when you find out something like this ... it's extremely hard," Poehl said. "These people are victims, too. They didn't know, they didn't expect and they surely didn't predict it. So, prayers to everyone in this whole mess."

Poehl said his client was in solitary confinement, but that it was normal for authorities to keep him there. He added that the attorneys would look at the issue of competency but, "We've got a lot of work to do before we can even speak intelligently about that."