Sources told ABC News the accused school shooter’s mother, Adriana Reyes, at one point worked at Oasis Outback, the same store where the gunman purchased two weapons just after his 18th birthday earlier this month.

The establishment is half gun retailer, half restaurant; Reyes’ employment was with the restaurant portion, sources say.

It is unclear if she had any role in her son’s purchase of the firearms. The owner of Oasis declined to comment to ABC News and added he would only speak with law enforcement at this time. Reyes has not responded to ABC News’ request for comment.

-ABC News' Matt Gutman, Laura Romero and Victor Ordonez

The response by school officials and law enforcement is becoming a key focus in the ongoing investigation into the Uvalde school shooting, law enforcement sources told ABC News Thursday.

It is unclear whether any students and teachers heard an official call for a lockdown once the gunman entered the building, the sources said.

Additionally, investigators are looking into whether officers on site could have made other attempts to enter the school to end the gunman’s rampage faster, the sources said. Responding police were met with gunfire and called for tactical teams with proper equipment to enter the classroom and neutralize the gunman, according to the sources.

-ABC News' Matt Gutman, Josh Margolin, Aaron Katersky and Luke Barr

A student who was in the classroom next door to the one the gunman entered recounted to ABC News what she did next.

Gemma Lopez, 10, said she heard five to six gunshots and commotion outside her classroom at Robb Elementary School before a bullet whizzed by her arm and into the wall. She recalled seeing a puff of smoke, which is when she knew they were all in danger.

She said she turned off the lights and then ducked under the tables -- what she learned to do in the active shooter training she has undergone since kindergarten. There were no locks inside and they did not have a key in the classroom to lock the door from the inside, she said.

Authorities yelled at the gunman to put down his weapon, to which he reportedly shouted in response, "Leave me alone please," in Spanish, Gemma recalled.

Gemma said her best friend, Amerie Jo Garza, was one of the 19 children killed in the massacre.

-ABC News' Matt Gutman and Olivia Osteen

The accused Robb Elementary School shooter appears to have sent disturbing messages -- including claims about intentions of violence at schools -- to numerous young people online in the days and weeks before the shooting, ABC News has found.

Over a dozen people told ABC News that the accused gunman, 18-year-old Salvador Ramos, sent them concerning messages across multiple social media platforms in the days leading up to the massacre.

Hours before the massacre, the gunman allegedly messaged a young girl on Instagram warning that he had a secret he wanted to tell her, according to messages reviewed by ABC News, which law enforcement sources say are part of the ongoing investigation into the shooter. He had tagged her in a photo of two guns days earlier.

In another alleged message the morning of the shooting, Ramos sent a photo of a gun laying on a bed, according to a user who shared direct messages from the suspect's alleged account with ABC News. She replied asking, “Why'd u send me a pic of a gun."

And moments before the attack, the accused gunman allegedly sent a string of messages to a young girl he met online, detailing that he had shot his grandmother and was heading to the school for his next target, according to messages reviewed by ABC News.

Another young user told ABC News the shooter was on Yubo the day before the massacre and implied something would occur the next day, the day of the shooting.

-ABC News' Will Steakin and Olivia Rubin

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.