“I told people, sometimes there aren’t any words,” said Melissa Cabralez, one of several counselors in town stepping in to help people who have been traumatized by the shooting. “We’re doing the best we can, but this is 21 families and extended families and a whole school campus of children who are scared.”

Immediately following the shooting, Gov. Greg Abbott of Texas downplayed the issue of gun control and emphasized the need to intervene before people become violent. “We as state, we as a society, need to do a better job with mental health,” he said.

Police officials said the gunman, Salvador Ramos, 18, who was killed by a law enforcement tactical team, had never been diagnosed with a mental illness, nor had he been arrested in connection with any crime.

Yet it has become clear that he had left plenty of warnings.

There was the video of him in the passenger seat of a car, holding a bag that appeared to contain a bloodied animal. There was the fight with another student in a high school bathroom, captured on camera, and the volatile arguments with his mother that a friend overheard when they played online video games together. There was the name he used within Fortnite: “Sal k1ll em all.”

There were also the violent thoughts he shared with girls around the world whom he met online. Sometimes the comments were addressed directly to the girls — he told one in Germany that he would “smack you in your face” if he was with her — and sometimes they made reference to an ex-girlfriend who, he said in one Instagram message, was “terrified of me.”

“I like hurting others,” he wrote last summer to a girl in San Jose, Calif., who is now 16 and asked that her name not be used. Seeing a photograph of his ex-girlfriend, he told her, had left him “feeling violent.” Two months later, he wrote to the same girl that he would rather see his ex-girlfriend “in a hospital with a broken neck” than to see her happy without him.

The fact that Mr. Ramos was able to legally buy two AR-15-style rifles shortly after his 18th birthday, along with hundreds of rounds of ammunition, has raised questions among some of those familiar with Mr. Ramos and his internet posts.