The 24-year-old National Youth Poet Laureate took to Twitter following the tragedy to share a poem.

She also posted a series of tweets on gun violence.

“It takes a monster to kill children,” she wrote. “But to watch monsters kill children again and again and do nothing isn’t just insanity -- it’s inhumanity.”

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President Joe Biden addressed the nation Tuesday night following the mass shooting in Texas -- not even two weeks after he mourned victims of the mass shooting in Buffalo, New York.

"I’d hoped, when I became president, I would not have to do this again," Biden said. "Another massacre. Uvalde, Texas. An elementary school. Beautiful, second-, third-, fourth-graders."

"As a nation, we have to ask when in God's name are we going to stand up to the gun lobby," he said. "I am sick and tired of it -- we have to act."

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Connecticut Democrat Chris Murphy gave an impassioned argument on the Senate floor to his colleagues late Tuesday afternoon, hours after 21 were killed in the mass shooting.

"What are we doing?" Murphy asked the chamber. "There have been more mass shootings than days in the year."

"Our kids are living in fear every single time they set foot in the classroom because they think they're going to be next. What are we doing?"

For Murphy's remarks and full coverage, click here.

A fourth-grade teacher and a 10-year-old boy were among those killed, ABC News has learned.

The teacher, Eva Mireles, had worked in the school district for approximately 17 years, her aunt, Lydia Martinez Delgado, confirmed to ABC News.

Fourth-grader Xavier Lopez was among the 19 children killed.

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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.