Fourth-grade teacher Irma Garcia was killed in her Texas classroom on Tuesday, massacred along with her co-teacher and 19 students. Two days later, a family member says her brokenhearted husband died.

The motive for the massacre — the nation's deadliest school shooting since Newtown, Connecticut, almost a decade ago — remained under investigation, with authorities saying the 18-year-old gunman had no known criminal or mental health history.

Shooter Salvador Ramos was inside the Robb Elementary School classroom in Uvalde for more than an hour before he was killed in a shootout, authorities said Thursday.

The rampage has rocked a country already reeling from gun violence, and the toll has continued to climb in Uvalde, a largely Latino town of some 16,000 people about 120km from the Mexican border.

Joe Garcia, the grief-stricken husband of fourth-grade teacher Irma Garcia who was killed in the Texas school massacre. Photo / news.com.au

Joe Garcia, 50, had dropped off flowers at his wife's memorial on Thursday morning, the New York Times reported. He "pretty much just fell over" after returning home and died of a heart attack, his nephew John Martinez told the newspaper.

The Archdiocese of San Antonio and the Rushing-Estes-Knowles Mortuary confirmed Joe Garcia's death to AP which was unable to independently reach members of the Garcia family on Thursday.

Archbishop Gustavo García-Siller will hold a Mass on Thursday evening at Sacred Heart Church in Uvalde for the Garcia family and the wider community, the archdiocese said.

Married for 24 years, the couple shared four children. The oldest, Cristian, is serving in the Marine Corps as his brother, Jose, attended Texas State University. Their eldest daughter Lyliana is a high school sophomore, while her younger sister is a seventh grade student.

Martinez, the Garcias' nephew, told the Detroit Free Press that the family was struggling to grasp that while Garcia's son trained for combat, it was his mother who was shot to death.

"Stuff like this should not be happening in schools," he told the newspaper. "It's wrong. It's not okay."

The Garcias loved to barbecue, 48-year-old Irma wrote in an online letter to her students at Robb Elementary. Irma enjoyed listening to music and taking country cruises to Concan, a community along the Frio River about 40.23km north of Uvalde.

A family pays their respects next to crosses bearing the names of the shooting victims at Robb Elementary School in Uvalde, Texas. Photo / AP

The school year, scheduled to end Thursday, was Irma's 23rd year of teaching — all of it at Robb Elementary School. She had been previously named the school's teacher of the year and was a 2019 recipient of the Trinity Prize for Excellence in Education from Trinity University.

"Mrs Irma Garcia was my mentor when I began teaching," her colleague Allison McCullough wrote when Garcia was named teacher of the year. "The wealth of knowledge and patience that she showed me was life changing."

For five years, Irma had co-taught with Eva Mireles, who also was killed.

"Welcome to the 4th grade! We have a wonderful year ahead of us!" Mireles wrote in an online letter to their incoming students last year.

Associated Press journalist Jamie Stengle in Dallas contributed.

- by Stefanie Dazio, AP