Texas school shooting: Suspect's grandmother upgraded to good condition

There were four Border Patrol agents, members of elite specialized units, who engaged and stopped the elementary school shooter, along with state and local officers, a Customs and Border Protection official told ABC News.

One agent, who was part of the Border Patrol Tactical Unit, or BORTAC, held a shield as the group pushed toward the gunman's incoming fire, the official said.

Two BORTAC agents discharged their weapons along with one member of the Border Patrol's Search, Trauma and Rescue unit -- all part of the group of four.

At least one BORTAC agent shot the gunman, although it's unclear who among them killed him, the official said.

-ABC News' Jack Date and Quinn Owen

Moments before carrying out the deadly attack at Robb Elementary, the suspected gunman allegedly sent a string of messages to a young girl he met online, detailing that he had shot his grandmother and was heading to the school for his next target, according to messages reviewed by ABC News.

The messages allegedly show the accused gunman, 18-year-old Salvador Ramos, texting with a 15-year-old in Germany who he met on the social media platform Yubo, describing an argument he was having with his grandmother over a phone bill before texting, "I shot my grandmother in the head" and "ima go shoot up a elementary school rn."

"She's on the phone with AT&T," Ramos allegedly wrote at 11:06 a.m., while referring to his grandmother with a derogatory term.

"Ima do something to her rn," he said in another alleged message. Minutes later, the gunman allegedly wrote, "I shot my grandmother in the head" before immediately adding "ima go shoot up a elementary school rn."

The friend did not reply to the accused gunman's messages until news broke regarding the shooting, according to the screenshot reviewed by ABC News.

Speaking to The New York Times, the girl said she only asked a friend in the United States about contacting law enforcement after she saw news of the shooting, telling The Times, “Maybe I could’ve changed the outcome... just could never guess that he’d actually do this.”

Law enforcement sources tell ABC News the messages are part of the ongoing investigation into the shooter.

-ABC News' Will Steakin, Josh Margolin and Aaron Katersky

March for Our Lives, a student-led movement in support of gun control legislation that formed in response to a 2018 school shooting, is planning a march in the nation's capital in the wake of the mass shooting at Robb Elementary School.

"In 2018 you marched with us to end gun violence. 4 years later, we're marching again," the organization said on Twitter Wednesday evening, announcing a march in D.C. on June 11.

The organization held a march following a shooting at Marjory Stoneman Douglas High School in Parkland, Florida, in 2018 that killed 17 people.

Authorities believe the Uvalde school shooter was shot and killed by a member of the Customs and Border Protection's tactical unit, known as BORTAC, CBP Del Rio Sector Chief Jason Owens told ABC News.

Owens, who did not identify the agent, said his actions "were absolutely courageous," but added it wouldn't be fair to single anyone out.

"It would be unfair to say that any one person's actions were singularly responsible for ending that threat," Owens said. "It took everybody."

-ABC News' Josh Margolin and Mireya Villarreal