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.”

President Joe Biden and first lady Jill Biden paid their respects at a memorial outside Robb Elementary School on Sunday, placing a bouquet of white roses and viewing and touching photos of the 19 children and two teachers killed in Tuesday's attack at the school.

The Bidens' motorcade arrived to applause from community residents lining the street nearby.

The president and first lady met Robb Elementary School principal Mandy Gutierrez. The president hugged Gutierrez and appeared to offer words of comfort.

Texas Gov. Greg Abbott also joined the Bidens at the memorial.