On the Morning He Died, Husband of Slain Texas Teacher Placed Flowers at Wife's Memorial

Video has surfaced from Thursday showing Joe Garcia, the grieving widower of Texas school shooting victim Irma Garcia, laying flowers at a memorial site for his wife mere hours before his own death.

PEOPLE was on the phone with John Martinez, Joe and Irma's nephew, when he learned of his uncle's death.

The couple had been married 24 years before Irma's murder Tuesday morning, at the hands of an 18-year-old gunman who walked into Robb Elementary School in Uvalde, Tx., with an assault rifle he'd recently purchased and opened fire in a classroom.

Irma, 46, and another teacher, Eva Mireles, were killed, as were 19 of their students.

Irma — a Robb Elementary School teacher for 23 years — first met Joe in eighth grade, and the two became high school sweethearts, Martinez said.

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The footage shows Joe — accompanied by an unknown man, seen patting his back in support — walking up to the 21 individual memorial crosses erected outside Robb Elementary School.

Joe places a bouquet of red roses in a vase at the cross bearing his wife's name and walks away. The pain on his face is visible; Joe can be seen towards the end of the footage taking in a deep breath and slowly exhaling.

It was unclear how long after visiting the memorial Joe died.

Martinez, 21, told PEOPLE on Thursday he believes Joe, 50, died "of a broken heart."

Martinez was told his uncle may have died from cardiac arrest, noting that chest compressions were performed on Joe in an effort to revive him.

The couple, he tells PEOPLE, had four children together.

When the gunman entered her classroom with a gun, Irma gave her life trying to save the children she loved like her own.

"She died a hero trying to protect her students," says Martinez, adding one of the 19 children killed in Tuesday's shooting was found dead in her arms. "She was a mom to them as well."

Adds Martinez: "She was a light in every room. She always had joy in her. She was a light to the world. I just really do want her to be remembered as someone who put her life on the line, to the very last moment. She sacrificed herself."

In 2019, Irma was named a finalist for the Trinity Prize, which honors local teachers chosen as distinguished educators by Trinity University in San Antonio.

According to her school profile, she loved to barbecue with Joe, and Martinez tells PEOPLE she loved classic rock.

A GoFundMe campaign launched to help cover the couple's funeral expenses had surpassed the $2 million mark Friday morning.

Prior to the massacre at the school, the shooter shot his grandmother in the face; the woman has so far survived her injuries.

Before entering the school, the killer crashed his car in a nearby ditch.

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All the fatalities took place inside one classroom, authorities have said.

A responding off-duty U.S. Customs and Border Patrol agent killed the suspect. A motive for the murders has not been released.

Police said at a Tuesday press conference they believe the shooter acted alone.

All of the children killed were in the second, third, and fourth grades, police have said.

The school district in Uvalde has opened an official account with First State Bank of Uvalde to support Robb Elementary families affected by the tragedy. People can send checks through the mail (payable to the "Robb School Memorial Fund") or donate money through Zelle to robbschoolmemorialfund@gmail.com. People can also donate by calling 830-356-2273.