The suspect’s family seemed to share that sentiment.

“Patrick’s actions were apparently influenced and informed by people we do not know, and from ideas and beliefs that we do not accept or condone, in any way,” the family said in a statement. “There will never be a moment for the rest of our lives when we will forget each and every victim of this senseless tragedy.”

[In photographs: In an empty parking lot behind the El Paso Walmart, dancers, mariachis and prayers commemorate the victims.]

Mark Stevens, a lawyer appointed to represent the suspect, declined to answer questions. “The prosecutors say they will try to put Patrick to death,” he said in a statement. “I will use every legal tool available to me to prevent that from happening.”

Neighbors and classmates described the suspect as “strange” and “off.” He was once seen by a neighbor playing outside with an ax. Another neighbor recalled that the suspect often gave terse “yes/no” answers and was often late for school because the clothes he wore did not feel right to him. That neighbor said the family rarely took vacations because the boy struggled with being in a car for long periods of time.

As a teenager, he briefly attended Liberty High School in the nearby suburb of Frisco, where his mother taught health science and medical terminology at the time. He later moved to Plano Senior High School, where he is not listed in the yearbook as a member of any club, sport or activity.

As a senior, he took a criminal justice class and enjoyed learning about “how the world of law enforcement works,” according to the yearbook. A former classmate said Mr. Crusius had expressed an interest in becoming a police officer, the classmate’s mother said.

To many fellow students, however, he was barely there. Some remembered him as an awkward, solitary presence in the hallways or English class, and said he did not seem to have many — if any — friends in a class of about 1,300 students.