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."

The Uvalde school shooting suspect's grandmother, who was the first person shot in Tuesday's attack, has been upgraded to fair condition, University Health officials tweeted Sunday.

The 66-year-old woman, who police said was allegedly shot in the head by her grandson, is being treated at University Hospital in San Antonio, officials said.

Two young victims who survived the shooting at Robb Elementary School remained in University Hospital on Sunday. A 10-year-old girl was listed in serious condition at the hospital, while a 9-year-old girl was listed in good condition, hospital officials said.

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.

The Justice Department will conduct a critical incident review to examine the law enforcement response to last week’s school shooting in Uvalde, Texas, a spokesperson announced on Sunday.

The review is being conducted at the request of Uvalde Mayor Don McLaughlin and the DOJ will publish a report on its findings at the conclusion.

"The goal of the review is to provide an independent account of law enforcement actions and responses that day, and to identify lessons learned and best practices to help first responders prepare for and respond to active shooter events. The review will be conducted with the Department’s Office of Community Oriented Policing," the DOJ said in a statement.

“As with prior Justice Department after-action reviews of mass shootings and other critical incidents, this assessment will be fair, transparent, and independent. The Justice Department will publish a report with its findings at the conclusion of its review.”

Those inside a classroom with the shooter made several calls to 911, but the tactical unit that arrived at 12:15 p.m. waited 35 minutes before breaching the classroom, Steven McCraw, director of Texas Department of Public Safety, said at a press conference Friday.

A 911 call was made at 12:03 p.m. from room 112 and lasted 23 seconds. McCraw did not identify the caller.

She called back at 12:10 p.m. and advised that there were multiple dead in the classroom, McCraw said.

The person then called again at 12:13 p.m. and again at 12:16 p.m., when said there were eight to nine students who were still alive, McCraw said.

A call was made by someone else from room 111 at 12:19 p.m., the caller hung up when another student told her to hang up, McCraw said.

At 12:21 p.m., three shots were heard over a 911 call. At 12:36 p.m., another 911 call was made by the initial caller and it lasted for 21 seconds. The student caller was told to stay on the line and be very quiet. She told 911 that the gunman shot the door, McCraw said.

At approximately 12:43 p.m. and 12:47 p.m., she asked 911 to please send the police now, McCraw said.

The caller said she could hear police next door at 12:46 p.m. At 12:50 p.m., the Border Patrol tactical unit finally breached the door and shot the suspect.