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.

A video interview with one student, Paige Curry, spread across social media, an artifact of a moment when children have come to expect violence in their schools.

“Was there a part of you that was like, ‘This isn’t real, this is — this would not happen in my school?’” the reporter asked.

The young girl shook her head: “No, there wasn’t.”

“Why so?” the reporter asked.

“It’s been happening everywhere,” she said. “I felt — I’ve always kind of felt like eventually it was going to happen here, too.”