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

As law enforcement officials continue to scrutinize each movement made by alleged gunman Salvador Ramos inside Robb Elementary School, Texas Department of Public Safety Director Steven McCraw said that law enforcement officers shot and killed the suspect roughly 40 minutes to an hour after his first encounter with the school district's resource officer at the building entrance.

"I don’t want to give you a particular timeline. But the bottom line is that law enforcement was there," McCraw told reporters during a briefing Wednesday. "They did engage immediately. They did contain him in the classroom, and they put a tactical stack together in a very orderly way and of course breached.”

McCraw later said that investigators plan to go frame-by-frame through surveillance footage to "track every minute" of the gunman's movement and will provide an update once that work is completed.

-ABC News' Lucien Bruggeman

When federal agents from Customs and Border Protection and Homeland Security Investigations arrived at Robb Elementary School Tuesday, they immediately encountered a scene of confusion and chaos, according to multiple law enforcement officials briefed on the attack.

Some arrived with heavy equipment, others with whatever they could carry as they ran to the scene on foot.

Initially, these agents, multiple law enforcement sources told ABC News, tried to discern who was doing what, where was shooter who was engaging law enforcement and how were children were being evacuated.

Once the tactical team assembled with enough gear, they became aware of an order not to enter the classroom because they were told the suspect had barricaded himself, the sources told ABC News.

The Texas Department of Public Safety has said the incident commander wrongly believed the shooting had stopped.

Eventually, the special agent-in-charge of Homeland Security Investigations gave the instruction to all federal agents under the umbrella of the Department of Homeland Security that they were free to use their best judgment and to do what they felt was best, the sources said.

The federal agents were unsure whether any children could be saved at that point, but they were interested in evacuating the wounded. Some agents brought children in other classrooms out through windows.

The tactical team went in at 12:50 p.m. CDT and fatally shot the suspect, 77 minutes after the shooting started, officials said.

The suspect was dead at the scene, the sources said. He was found with more than a dozen bullet wounds.