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.
MORE: What we know about school shooting suspect
Sophomore Kyle Harris, who took part in the walkout last month, was in first period when a fire alarm went off. Then, he heard teachers urging him to flee.
RELATED: How the deadly school shooting unfolded
"The scariest thing is hearing a teacher who knows your name personally call you by your name and tell you to run," Harris tweeted.
VIDEO: Students escorted out by law enforcement
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.
MORE: Student's first person witness account
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.
We grieve for the terrible loss of life, and send our support and love to everyone affected by this horrible attack in Texas. To the students, families, teachers and personnel at Santa Fe High School – we are with you in this tragic hour, and we will be with you forever... pic.twitter.com/LtJ0D29Hsv— Donald J. Trump (@realDonaldTrump) May 18, 2018
The governor 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.
"It's been happening everywhere. I've always kind of felt like that eventually it was going to happen here too," Santa Fe High School student Paige Curry told Houston television station KTRK. "I don't know. I wasn't surprised. I was just scared."
Although most of the dead had not been publicly identified, relatives and acquaintances said the fatalities included a substitute teacher and a foreign exchange student from Pakistan.
Another 10 people were wounded at the school in Santa Fe, a city of about 13,000 people roughly 30 miles (48 kilometers) southeast of Houston, the governor said. The wounded included a school police officer who was the first to confront the suspect and got shot in the arm.
Hospitals reported treating a total of 14 people for injuries related to the shooting. Two were listed in critical condition Saturday at the University of Texas Medical Branch, the hospital tweeted.
Zachary Muehe, a sophomore at the school of roughly 1,400 students, was in his art class when he heard three loud booms.
Muehe told The New York Times that a student he knew from football was armed with a shotgun and was wearing a shirt emblazoned with the slogan "Born to Kill."
"It was crazy watching him shoot and then pump. I remember seeing the shrapnel from the tables, whatever he hit. I remember seeing the shrapnel go past my face," he told the Times.
Michael Farina, 17, heard the fire alarm and thought it was a drill. He was holding a door open for special education students in wheelchairs when a principal came bounding down the hall, telling everyone to run. Another teacher yelled out, "It is real!"
Students were led to take cover behind a car shop across the street from the school. Some still did not feel safe and began jumping the fence behind the shop to run even farther away, Farina said.
"I debated doing that myself," he said.
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. A woman who answered the phone at a number associated with the Pagourtzis family declined to speak with the AP.
Pagourtzis made his initial court appearance Friday 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.
According to Pagourtizis' lawyers, his parents have said the guns were locked up and they did not know that their son was capable of violence.
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.
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.
On Saturday, Santa Fe Independent School District released a letter that said crisis intervention and support services will be put in place for students and victims' families.
ABC7 Chicago contributed to this report.