SANTA FE, Texas -- Only weeks ago, a dozen students from Santa Fe High School in Texas offered support for survivors of the Parkland, Florida, school shooting by participating in a nationwide walkout seeking stricter gun control. On Friday, it was Parkland students who declared their solidarity with teens in Santa Fe after a 17-year-old armed with a shotgun and a pistol opened fire at the Houston-area school, killing 10 people.

It was the nation's deadliest such attack since the Florida massacre that killed 17 and energized the teen-led gun-control movement.

Sophomore Kyle Harris, who took part in the walkout last month, was in first period when a fire alarm went off Friday. Then, he heard teachers urging him to flee.

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"The scariest thing is hearing a teacher who knows your name personally call you by your name and tell you to run," Harris tweeted.

The suspected shooter, who was in custody on murder charges, also had explosive devices that were found in the school and nearby, said Gov. Greg Abbott.

Investigators offered no motive. In a probable cause affidavit, however, authorities said the suspect admitted to the shooting. The gunman also told investigators that when he opened fire Friday morning, "he did not shoot students he did like so he could have his story told," according to the affidavit.

Abbott said the assailant intended to kill himself but gave up and told police that he did not have the courage to take his own life.

The deaths were all but certain to re-ignite the debate over gun regulations, coming just three months after the Florida attack.

Santa Fe High School student Paige Curry said she wasn't surprised a shooting unfolded.

"It's been happening everywhere," she said. "I felt -- I've always kind of felt like eventually it was gonna happen here, too. So I don't know. I wasn't surprised, I was just scared."

Fleeing students were huddled onto fields away from the school as frightened parents raced to the scene looking for terrified children. Ten people who were wounded were rushed to area hospitals. Six students were taken to Clear Lake Regional Medical Center.

The wounded included a school police officer who was the first to confront the suspect and got shot in the arm. The Houston police chief said Friday the officer, John Barnes, was "hanging in there."

"He was bleeding out and his blood pressure was very low, so the immediate issue was to resuscitate him and give him enough blood product to get his blood pressure up and take him straight to the OR," said a surgeon.

The gunman yelled "Surprise" before he started shooting, according to Texas Rep. Michael McCaul, the chairman of the House Homeland Security Committee.

The suspect was identified as Dimitrios Pagourtzis, who appeared to have no prior arrests or confrontations with law enforcement. He made his initial court appearance Friday evening by video link from the Galveston County Jail. A judge denied bond and took his application for a court-appointed attorney.

McCaul, a former federal prosecutor, said he expects the Justice Department to pursue additional charges, possibly involving weapons of mass destruction.

Pagourtzis played on the junior varsity football team and was a member of a dance squad with a local Greek Orthodox church. Acquaintances described him as quiet and unassuming, an avid video game player who routinely wore a black trench coat and black boots to class.

The suspect obtained the shotgun and a .38-caliber handgun from his father, who owned them legally, Abbott said. It was not clear whether the father knew his son had taken them. Investigators were determining whether the shotgun's shortened barrel was legal, Texas Sen. John Cornyn said.

The assailant's homemade explosives included pipe bombs, at least one Molotov cocktail and pressure-cooker bombs similar to those used in the Boston Marathon attack, authorities said.

While cable news channels carried hours of live coverage, survivors of the Feb. 14 Florida attack took to social media to express grief and outrage.

"My heart is so heavy for the students of Santa Fe High School. It's an all too familiar feeling no one should have to experience. I am so sorry this epidemic touched your town - Parkland will stand with you now and forever," Marjory Stoneman Douglas student Jaclyn Corin said in a tweet.

She also directed her frustration at President Donald Trump, writing "Our children are being MURDERED and you're treating this like a game. This is the 22nd school shooting just this year. DO SOMETHING."

In Texas, senior Logan Roberds said he was near the school's art room when he heard the fire alarm and left the building with other students. Once outside, Roberds said, he heard two loud bangs. He initially thought somebody was loudly hitting a trash can. Then came three more bangs.

"That's when the teachers told us to run," he said.

At that point, Roberds said, he told himself, "Oh my God, this is not fake. This is actually happening."

Roberds said he doesn't think additional gun-control measures are needed, citing the need for defense against intruders.

"What are you going to do if some guy comes in your house and points a gun at you? You can't do nothing with a knife," he said.

Friends and family attend a vigil held at the First Bank in Santa Fe for the victims of a shooting incident at Santa Fe High School, May 18, 2018. Bob Levey/Getty Images

Friday's assault was the deadliest in Texas since a man with a semi-automatic rifle attacked a rural church late last year, killing more than two dozen people.

There were few prior clues about Pagourtzis' behavior, unlike the shootings in Parkland and the church in Sutherland Springs, Abbott said, but the teen wrote in journals of wanting to carry out such an attack and then to end his own life.

"This young man planned on doing this for some time. He advertised his intentions but somehow slipped through the cracks," Cornyn said.

In the aftermath of the Florida assault, survivors pulled all-nighters, petitioned city councils and state lawmakers, and organized protests in a grass-roots movement. Within weeks, state lawmakers adopted changes, including new weapons restrictions.

In late March, the teens spearheaded one of the largest student protest marches since Vietnam in Washington and inspired hundreds of other marches from California to Japan.