Abbott said that the gunman apparently carried out the attack with a shotgun and a .38 revolver belonging to his father, and that the weapons were legally owned.

The Houston Chronicle reported that Pagourtzis was armed with pipe bombs in addition to firearms when he burst into a classroom at the start of the school day.

Abbott said searches were being conducted at two residences and “various explosive devices” were found including a “CO2 device” and a Molotov cocktail.

He said journal entries by the gunman suggested that he wanted to commit suicide but that “he gave himself up”.

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Abbott also said that there were no “warning signs” about the suspect ahead of time although he did post a picture on his Facebook page of a T-shirt with the words “Born to Kill” on it.

Law enforcement authorities were questioning two people including one who “may have had some level of involvement in the crime,” Abbott added.

Harris County Sheriff Ed Gonzalez said the “majority” of the dead were students.

“Officers inside encountered a bloody mess in the school,” a senior law enforcement official told the Chronicle. “Evidently this guy threw pipe bombs all in there. We don’t know if any of them went off.”

Area hospitals reported that two victims were in critical condition. One of the wounded was a police officer.

The shooting was the latest in what has become an all-too-familiar situation in American schools, where the threat of gun violence has shadowed everyday life.

“My friend got shot in the art hall,” a weeping Santa Fe student, Dakota Shrader, told reporters.

“I shouldn’t be going through this,” Shrader said as her mother comforted her. “It’s my school. This is my daily life. I feel scared to even go back.”

Shrader said that fire alarms went off “and everybody just started running outside.

“And the next thing you know everybody looks and you hear ‘boom, boom, boom’ and I just ran as fast as I could to the nearest forest so I could hide and I called my mom.”

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Earlier this year, 17 students and staff were killed at a Florida high school – a massacre that prompted survivors to launch a grass roots campaign against gun violence.

A sombre US President Donald Trump expressed “sadness and heartbreak” over the latest shooting spree.

“This has been going on too long in our country,” said the president, speaking at an event on prison reform at the White House. “We’re with you in this tragic hour.”

Trump, who has previously shied away from the topic of gun control and proposed arming teachers, said that his administration was “determined to do everything in our power to protect our students, secure our schools, and to keep weapons out of the hands of those who pose a threat to themselves, and to others.”

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The shooting erupted before 8am, as the first classes of the day were starting.

Student Paige Curry told local media that she heard gunshots and then a fire alarm, after which students were taken out by teachers to a nearby service station.

“I saw some girl – she had, you know, she got shot in the kneecap, I guess. So she had a bandage around it. She was limping and then the firefighters came and got her,” another student, Tyler Turner, told CBS.

The school district, which quickly imposed a lockdown, cleared the students from the campus, with television footage showing them leaving in a single file.

Parent Richard Allen told KTRK-TV he arrived at the school soon after the shooting and reported seeing a number of victims being taken away in ambulances.

“My son said someone went into the art room and started shooting a lot of the kids,” Allen said.

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The Santa Fe assault comes three months after the shooting at Marjory Stoneman Douglas High School in Parkland, Florida, which sparked a student-led movement for tighter gun laws.

“We are fighting for you,” tweeted David Hogg, a Stoneman Douglas student who has emerged as a leader of the gun-control campaign.

“We cannot let this continue to be the norm,” tweeted Delaney Tarr, another student at Stoneman Douglas. “We cannot.”