The gunman warned about the shooting in private messages on Facebook minutes beforehand, a spokesperson for Meta said Wednesday. The warning on Facebook was made in "private one-to-one text messages that were discovered after the terrible tragedy occurred."

The gunman sent three messages on Facebook about 30 minutes before the shooting saying: "I'm going to shoot my grandmother," "I shot my grandmother" and "I'm going to shoot an elementary school," Texas Gov. Greg Abbott said at a press conference.

Abbott said the suspect was reportedly a high school dropout. Officials have reportedly not identified a criminal history for the suspect, but Abbott said he may have had a juvenile record, but that is yet to be determined.

Abbott said the suspect did not have a known mental health history.

According to the ATF, the shooter made his gun purchases on two separate days: May 17 and May 20.

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

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.