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

The suspected Robb Elementary School shooter was not wearing body armor during Tuesday's massacre, multiple law enforcement officials told ABC News.

The alleged gunman was wearing tactical gear, including a vest that could hold ammunition, but there was no armor or fabric that would protect him from gunfire, the sources said.

-ABC News' Matt Gutman, Josh Margolin and Aaron Katersky

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.