Beto O'Rourke, a candidate running for governor of Texas, interrupted a press conference held by current Gov. Greg Abbott Wednesday, shouting at Abbott before being escorted from the building.

O'Rourke protested the current governor's stance on gun control.

"You are doing nothing!” he shouted.

Six people injured in the Texas elementary school shooting remain hospitalized on Wednesday.

Three children and one adult are at the University Hospital in San Antonio, two of whom are in serious condition.

Two other adults are hospitalized at Brooke Army Medical Center, both in serious condition.

All patients treated at Uvalde Medical Center have been discharged. The hospital said it treated 15 individuals, 11 of whom were children. Three of those 11 children were transferred to other hospitals in San Antonio and eight were discharged home.

The four remaining patients were adults, one was transferred to another hospital and three were discharged home.

-ABC News' Jennifer Watts

Jacinto Cazares, the father of Jacklyn Jaylen Cazares, a 10-year-old killed in the Texas elementary school shooting, told ABC News he blames the school and police for the massacre, not the murder weapons.

"I want to say we hold the school responsible for not having locked doors and no protocol, no training. Also the Police for not having a better and faster tactical response time. There was at least 40 lawmen armed to the teeth, but didn't do a darn thing till it was far too late," Cazares told ABC News in a statement.

He added, "The situation could've been over quick if they had better tactical training and we as a community witnessed it first hand. I'm a gun owner and I do not blame the weapons used in this tragedy. I'm angry how easy it is to get one and young you can be to purchase one."

-ABC News' Miles Cohen

Secretary of Homeland Security Alejandro Mayorkas said the department is "horrified by this callous act of violence," in a statement Wednesday.

Mayorkas said one Border Patrol agent was injured in the crossfire.

"We are grateful for the courageous members of our Border Patrol, many of whom are part of the Uvalde and surrounding communities, who immediately responded to the scene along with local and state law enforcement," Mayorkas said.

He added, "Without hesitation, they put themselves between the shooter and students to end the bloodshed and administer medical aid. Without question, their heroism yesterday saved lives."

The department will continue to coordinate with local, state and federal partners, Mayorkas said.

"As we pray for the families and loved ones and recognize the bravery of frontline law enforcement personnel, we must redouble our collective efforts to make our communities safer," Mayorkas 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.