SANTA FE, Tex. — A nation plagued by a wrenching loop of mass school shootings watched the latest horror play out in this small Southeast Texas town Friday morning, as a young man armed with a shotgun and a .38 revolver smuggled under his coat opened fire on his high school campus, killing 10 people, many of them his fellow students, and wounding 10 more, the authorities said.
By the end of the day, a 17-year-old suspect, Dimitrios Pagourtzis — an introvert who had given off few warning signs — had surrendered and been taken into custody. Law enforcement officials said they found two homemade explosive devices left at the school during the rampage.
It was the worst school shooting since the February assault on Marjory Stoneman Douglas High School in Parkland, Fla., where a young man with an AR-15 rifle left 17 people dead and prompted a wave of nationwide, student-led protests calling on lawmakers to tighten gun laws.
It was barely after 7:30 a.m. at Santa Fe High School, about 35 miles southeast of Houston, when gunfire first resounded through the halls, the opening volley of yet another massacre at an American high school that would leave students, teachers and staff members shocked, and in some cases bloodied. But they were not necessarily surprised.
President Trump, in the East Room of the White House, expressed his solidarity with the people of Santa Fe, and said his administration would do “everything in our power” to protect schools and keep guns away from those who should not have them.
Mr. Trump had also vowed to take action after the Parkland shooting. At the time, the president, a member of the National Rifle Association who has strong political support from gun owners, said he would look at stricter background checks and raising the minimum age for buying an assault weapon, proposals that the group opposes.
He also pressed for an N.R.A.-backed proposal to arm teachers, and said he would favor taking guns away from potentially dangerous people.
But Mr. Trump did not press for action on any of those initiatives, and Congress did not follow through. Attorney General Jeff Sessions said on Friday that the Justice Department was proposing to ban so-called bump stocks through regulations rather than wait for Congress to act.
The authorities had not released the names of those who died in the shooting late Friday, but family and associates of some of the victims had begun to share their stories on social media. The family of Cynthia Tisdale, a teacher, said on Facebook that she had been killed in the shooting. And on the Facebook page of the Pakistani Embassy in Washington, Pakistan’s ambassador to the United States expressed condolences for the victims, which he said included a Pakistani exchange student named Sabika Sheikh.
The shooting in Texas began at the start of a school day when summer seemed just around the corner. The night before, seniors had gathered for a sunset dinner and a Powder Puff football game, according to the school’s website, and the baseball team had been playing in the regional quarterfinals.
Zachary Muehe, a sophomore, headed to school thinking about the late work he was supposed to submit before the end of the school year, and settled into his art class to work on a drawing project. He was engrossed in his phone, he said, when his class began to transform into a horror scene.
It started with a boom, and then one or two more. “I turned around and I saw the kid who’s in my football class, I see him every day, and I saw him with a shotgun,” Mr. Muehe said in a phone interview. “I saw him in a trench coat. My immediate thought was just get out.”