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?"

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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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At least 21 people are dead after a gunman opened fire in Uvalde on Tuesday, authorities said.

Most of the victims were children in their last week of school before the summer break. Many other students and adults were injured in the mass shooting.

Here's what we know about what unfolded so far.

As President Joe Biden left Mass at a Catholic Church in Uvalde, Texas, on Sunday afternoon, he walked over to a crowed of people outside chanting, "Do something."

Biden looked at the crowd and mouthed the words, "We will."

Biden and first lady Jill Biden attended Mass at Sacred Heart Catholic Church. As they entered the sanctuary, Jill Biden reached out and lightly touched the hands of several people seated along the aisle as she and the president made their way to their seat in the first-row pew.

About 600 people attended the service.

The president and first lady were greeted at the church by Archbishop Gustavo Garcia-Siller, according to the White House.

“In the midst of collapse and devastation, we have come once more to this our house. To pray. And to be together," one of the service leaders said as the Mass got underway.

Garcia-Siller invited children to come to the front of the church and sit.

"They passed away. But you are alive," the archbishop told the children, referring to the 19 students and two teachers killed in the attack at Robb Elementary School on Tuesday.

Garcia-Siller told the children that he believes they will help the community heal. He noted how Jesus died and “rose from the dead," and then went to heaven.

“He prepared a place for the 19 little ones we lost,” the archbishop said.

The archbishop said he has experienced the goodness of people in the wake of the horrific tragedy. He reflected on Jesus’ message of peace, saying, Jesus felt fear and pain, but did not become bitter.

Garcia-Siller spoke of the joy of heaven.

“Our children in Uvalde who left us already join in this joy," he said. “Heaven is open for all."