Image copyright Galveston County Jail/Twitter Image caption The suspect has been booked into the Galveston County Jail

He was in a church dance club. He played on the school football team. He was a high-achieving student. And yet he allegedly opened fire on classmates, killing 10 people.

Officials say there were few red flags from Dimitrios Pagourtzis, the 17-year-old facing capital murder charges over Friday's Santa Fe High School shooting in Texas.

Texas Governor Greg Abbott said a photo on the suspect's now-deleted Facebook page showing a T-shirt with the phrase Born to Kill may be the only warning sign.

"But as far as investigations by law enforcement agencies, as far as arrests or confrontation with law enforcement, as far as having a criminal history, he has none," he told a news conference.

"His slate is pretty clean. There simply were not the same type of warning signs that we've seen in so many other shootings."

Image copyright Dimitrios Pagourtzis/Facebook Image caption The 17-year-old suspect, Dimitrios Pagourtzis

However, hours before he allegedly stormed into an art class armed with a shotgun and revolver the teenager made a weird post on social media, a law enforcement source told CBS News.

Accompanied by an occult symbol, it said simply, "Dangerous Days".

He had also previously posted an image of a trench coat pinned with various insignia, including the Iron Cross used by the Nazis, which the teen wrote represented "bravery".

Media playback is unsupported on your device Media caption How the Texas school shooting unfolded

The Communist hammer and sickle pin, he said, stood for "rebellion", and a depiction of the idol Baphomet symbolised "Evil".

Student Dustin Severin told KPRC-TV that he saw the teen in the hall before the shooting wearing his usual outfit of black boots and a trench coat.

He said the suspect had been picked on by school coaches "for smelling bad", and had mostly kept to himself.

One of his teachers told the New York Times: "He was quiet, but he wasn't quiet in a creepy way."

Police say the teenager detailed his plans to carry out the school shooting in a diary, on his computer and on a mobile phone.

Image copyright Facebook/ police handout

The suspect had planned to take his own life, say investigators, but he ultimately gave himself up.

And yet there were many other signs that Dimitrios Pagourtzis was a regular, outgoing teenager full of promise.

School officials say he was previously on the school's "honour roll" of high-achievers, and was expected to graduate in 2019.

According to local media, he was a member of a dance squad with a local Greek Orthodox church.

He had also played for the Santa Fe High School Indians American football team for the 2015-16 season.

Such wholesome extracurricular activities only add more emphasis to the question bewildered members of his community are asking in the aftermath of the rampage:

Why?