Authorities shed more light on some of the suspect's digital footprint in the weeks and months before Tuesday's mass shooting.

In September 2021, suspected gunman Salvador Ramos asked his sister to buy him a gun and she "flatly refused," Steven McCraw, director of the Texas Department of Public Safety, told reporters during a briefing Friday. He did not note where this exchange took place.

On Feb. 28, in an Instagram group chat with four people, they discussed "Ramos being a school shooter," McCraw said.

The next day, on March 1, in an Instagram chat with four people, Ramos discussed buying a gun, according to McCraw.

Two days later, on March 3, in another four-person chat, someone said, "Word on the street is you are buying a gun," according to McCraw. Ramos reportedly replied, "Just bought something."

On March 14, Ramos posted on Instagram, "Ten more days," according to McCraw. Someone replied, "Are you going to shoot up a school or something?" to which Ramos replied, "No. Stop asking dumb questions. You'll see," according to McCraw.

Meta said it is working with law enforcement regarding the concerning messages sent on Instagram.

"The messages Gov. Abbott described were private one-to-one text messages that were discovered after the terrible tragedy occurred," Meta spokesperson Andy Stone said on Twitter. "We are closely cooperating with law enforcement in their ongoing investigation."

Investigators are also looking into people the suspect may have communicated with in video game chat rooms who "may have known something," McCraw said.

Steven McCraw, director of Texas Department of Public Safety, admitted it was the "wrong decision" for officers not to go into the classroom where the suspect was for 35 minutes after a tactical unit arrived. Children were inside the classroom with him, making 911 calls, McCraw said in a press conference Friday.

The incident commander believed he was dealing with a barricaded subject inside the school and the children were not at risk, he said.

A tactical team from Customs and Border Protection was on scene at 12:15 p.m., but did not breach the classroom until 12:50 p.m.

"Of course it wasn't the right decision," McCraw said. "It was the wrong decision."

The U.S. Marshals said they never placed anyone in handcuffs, but they say they “maintained order and peace in the midst of the grief-stricken community that was gathering around the school," in a statement posted on Twitter.

U.S. Marshals arrived on scene from Del Rio, Texas, at 12:10 p.m., and the first deputy U.S. Marshal went into the school to assist BORTAC, the elite tactical CBP team that ultimately shot the alleged shooter, the statement said.

They came from 70 miles away and got the first call around 11:30 a.m., according to the statement.

“These Deputy US Marshals also rendered emergency trauma first aid for multiple victims,” the statement said.

"Additional Deputy U.S. Marshals were asked to expand and secure the official law enforcement perimeter around the school,” the statement said. “Our hearts are heavy with sorrow and sadness at this horrific crime. We send our condolences to all the victims and families affected by this tragedy."

Angeli Rose Gomez, a mother waiting outside for her children, told the Wall Street Journal she was one of numerous parents urging police and law enforcement officers to go into the school sooner, first politely and then more urgently. She said U.S. Marshals put her in handcuffs, and told her she was being arrested for intervening in an active investigation.

Angel Garza, the stepfather of one of the children killed in the shooting, ran to try to reach and help his child, and was restrained and handcuffed by a local police officer, Desirae Garza, the girl's aunt, recounted to the New York Times.

There was blood in the hallway and children were covered in it, one of the students who survived the mass shooting at Robb Elementary School in Uvalde, Texas, told ABC News.

Samuel Salinas was a student in Irma Garcia's fourth grade class. They were scheduled to graduate Thursday, but the ceremony was canceled because Garcia, another teacher and 19 third and fourth grade students were killed in Tuesday's massacre.

Salinas said his aunt dropped him off for school on Tuesday morning.

"It was a normal day until my teacher said we're on severe lockdown," he told ABC News, "and then there was shooting in the windows."

Salinas said the gunman came into his classroom, closed the door and told them, "You're all going to die," before opening fire.

"He shot the teacher and then he shot the kids," Salinas said, recalling the cries and yells of students around him.

-ABC News' Samira 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.