The dead include some adult staff in addition to students, Harris County Sheriff Ed Gonzalez said.

“There have been explosive devices found in the high school and surrounding areas adjacent to the high school,” the school district said in a tweet. “Because of the threat of explosive items, community members should be on the lookout for suspicious packages and anything that looks out of place.”

Law enforcement officials speaking to national and local news outlets identified the gunman as Dimitrios Pagourtzis, a junior at the school who was set to graduate in 2019 and who has previously played football, according to the district’s website.

Recent social media posts made under Pagourtzis’ name revealed a fascination with guns, according to Facebook and Instragram accounts reviewed by the Los Angeles Times.

On April 30, in a Facebook profile that has since been removed, a user under Pagourtzis’ name posted an image of a T-shirt bearing the phrase “BORN TO KILL.” Another posting the same day showed a black trenchcoat with a variety of occult and World War II pins on it.

“Hammer and Sickle=Rebellion, Rising Sun=Kamikaze Tactics, Iron Cross=Bravery, Baphomet=Evil, Cthulu=Power,” the caption said.

“All I have to say is dimitri wore a trench coat to school every day from the beginning of the year and it’s no wonder he was capable of getting guns in the school,” Santa Fe High junior Candi Thurman, 17, wrote to The Times in a private message on Twitter on Friday. “That should have been a red flag.”

In the Facebook profile, which prominently featured a pentagram, the user described himself as an “atheist” and declared, “I hate politics.”

An Instagram profile under Pagourtzis’ name shows a posting from April 24 with a photo of a handgun, a knife and what appears to be a flashlight. Another post from the same day showed an arcade game featuring a toy rifle pointing at a screen.

The Instagram account followed only a handful of other accounts, mostly geared toward gun users, along with Instagram accounts for President Trump, Melania Trump, Ivanka Trump and the White House.

On Friday from Washington, President Trump described the shooting as a “horrific attack” and offered his condolences .

“This has been going on too long in our country — too many years, too many decades now,” Trump said. “We grieve for the terrible loss of life and send our support and love to everyone affected by this absolutely horrific attack to the students, families, teachers and personnel at Santa Fe High.”

Texas Gov. Greg Abbott ordered flags in the state to be flown at half-staff. The shooting comes less than a year after the region was barraged by Hurricane Harvey in August 2017 and after a separate mass shooting left 26 people dead at a San Antonio-area church in Sutherland Springs, Texas.

Paige Curry, a junior, said she was sitting in her classroom when she heard really loud booms. At first, she was not sure what they were. Then she heard screaming.

“I got up and I ran,” Curry told KPRC, a local television station, saying she hid back stage with fellow students as alarms started going off.

“I was very, very scared,” she said. “I had to have someone keep me calm. I managed to keep calm through it all. There was another girl who was just freaking out. They were struggling really hard to keep her calm. It was really scary.”

After so many mass shootings reported on the news, she said she was not surprised to witness her school become a crime scene.

“It’s been happening everywhere,” she said. “I’ve always kind of felt that eventually it was going to happen here, too. So I don’t know. I wasn’t surprised. I was just scared.”

Stephanie Skillman, a 17-year-old junior, was in the middle of first period when she heard the fire alarm go off. She evacuated to a grassy area outside the school and was waiting with about 100 other students when she first heard gunshots — about five of them.

“The fire alarm is the reason the school evacuated and whoever pulled the fire alarm saved my life,” Skillman wrote The Times in a direct message on Twitter.

The shooting prompted expressions of grief, anger and sympathy from Parkland, Fla., students and parents of victims, who took to social media to call for government action on guns and to offer support to the school in Texas.

“Students of Santa Fe, I lost my sister in February cause of the reason you lost your friends.” Hunter Pollack wrote on Twitter. “I know exactly what you’re feeling right now, and this makes me want to fight even more for you guys. Stay strong.” Pollack’s sister, Meadow, was 18.

Her father, Andrew Pollack, also sent out a tweet. “Nothing has changed to protect our kids. We need solutions not rhetoric. We need single point entry and metal detectors. No one should be able to walk into a school with a gun,” he wrote.

Stoneman Douglas student Emma Gonzalez, who has emerged as a vocal gun-control advocate since the shooting, tweeted: “Santa Fe High, you didn’t deserve this. You deserve peace all your lives, not just after a tombstone saying that is put over you. You deserve more than Thoughts and Prayers, and after supporting us by walking out we will be there to support you by raising up your voices.”

Another student, Lauren Hogg, described learning of the Santa Fe shooting while on the Stoneman Douglas campus. “Today when I read this news my heart immediately broke,” she wrote on Twitter. “As I looked up from my phone I realized that I myself was standing in the same hallway that I was in just three months ago when I was told to put my hands above my head and not to move. How many more?”

Fred Guttenberg, who lost his 14-year-old daughter, Jaime, in the Parkland shooting said the latest attack underscored the need for gun-control.

“My daughter’s murder happened just over three months ago, and the fact that our President, our House speaker, and our Senate majority leader still can’t talk about guns is pathetic,” he said in a telephone interview. “Enough is enough. It is time for them to get off their asses and to do something about this before the next mass casualty happens. It’s just unacceptable.”

Staff writer Molly Hennessy-Fiske contributed from Harlingen, Texas and special correspondent Jenny Jarvie contributed from Atlanta.

UPDATES:

12:05 p.m.: This article was updated with information on the alleged gunman and additional reaction.

8:15 a.m.: This article was updated with local media reporting fatalities from the shooting.

7:45 a.m.: This article was updated with Assistant Principal Cris Richardson reporting the suspect’s arrest and with more details from the school district.

7 a.m.: This article was updated with more details about the scene and information on a lockdown in February at the school.