A timeline of how the Texas school massacre -- and the police response -- unfolded

As a broken community tries to make sense of a massacre that took the lives of 19 young children and two teachers, authorities have offered shifting timelines of what happened inside the Uvalde, Texas, school.

On Friday, Texas Department of Public Safety Director Steven McCraw laid out the most detailed breakdown the public has received to date about the horror that unfolded in Robb Elementary School on May 24 -- and attempted to offer some answers about the way authorities responded.

Among the details we know now are: that a school officer drove right past the shooter -- 18-year-old Salvador Ramos -- while Ramos fired at the school; that as many as 19 officers were inside the school more than 45 minutes before the suspect was killed; that the school district police chief decided not to breach the classroom where the shooter was; and that a young girl from the class called 911 several times asking for police while authorities were right outside.

CNN created a timeline of events with information provided by McCraw, social media posts and other reporting that offers a look into what came before the shooting and a minute-by-minute breakdown into the attack -- and how authorities responded to it.

• 10-year-old boy recounts 'horrifying experience'
• She smeared blood on herself and played dead: 11-year-old reveals chilling details of the massacre
• WATCH: Merella Fernandez on families coping with loss in Texas

IN THE MONTHS PRIOR

In September 2021, the shooter asked his sister to help him buy a gun and she "flatly refused," McCraw said.

The shooter was in a group chat on Instagram and in it, there was a February 28 discussion of the suspect being a "school shooter," McCraw said.

On March 1, the shooter had an Instagram chat with several others in which he discussed buying a gun, McCraw said. Two days later, there was another group chat in which someone said, "word on the street" was that the suspect was buying a gun. The shooter replied, "just bought something rn."

On March 14, the shooter wrote in an Instagram post, "10 more days." Another user replied, "'are you going to shoot up a school or something?' The shooter replied, 'no and stop asking dumb questions and you'll see,'" McCraw said.

On May 17 and May 20, the shooter legally purchased two AR platform rifles at a local federal firearms licensee, said Texas state Sen. John Whitmire, who received a briefing from law enforcement.

The shooter also purchased 375 rounds of ammunition on May 18, Whitmire said, citing law enforcement.

State Sen. Roland Gutierrez said the purchases were made for the suspect's 18th birthday.

HE SHOT HIS GRANDMOTHER AND LEFT THE HOUSE

Before going to the school and committing a massacre, the shooter sent a series of chilling text messages to a girl he met online, according to screenshots reviewed by CNN and an interview with the girl.

The teen girl, who lives in Germany, said she began chatting with the shooter on a social media app earlier this month. The shooter told her that on Monday, he received a package of ammunition, she said.

On Tuesday morning, Ramos called her and told her he loved her, she said.

He complained about his grandmother being on the phone with AT&T about "my phone."

"It's annoying," he texted.

Six minutes later, at 11:21 a.m., he texted: "I just shot my grandma in her head."

Seconds later, he said, "Ima go shoot up a(n) elementary school rn (right now)."

SUSPECT BEGAN FIRING OUTSIDE OF SCHOOL

On Tuesday, May 24, at 11:27 a.m.: Video shows an exterior door of Robb Elementary school, which is suspected to be where the shooter entered from, was propped open by a teacher, McCraw said during a Friday news conference.

11:28 a.m.: The shooter's vehicle crashes into a ditch near the school. A teacher runs into a room to get a phone and returns to the exit door, which remains propped open, McCraw said. The suspect jumps out from the passenger side of the truck with a rifle and a bag, which officials later discovered was ammunition, DPS director Victor Escalon said at a Thursday news conference.

Two witnesses meanwhile, who were at a funeral home across the street from the school, hear the crash and go to see what happened. The suspect begins firing at them, McCraw said. Both began running away.

11:30 a.m.: A teacher reemerges in a panic and "apparently" called 911, McCraw said. In a separate statement, the US Marshals Service said it received a call for assistance at 11:30 a.m. from a Uvalde Police Department officer.

11:31 a.m.: The suspect reaches the last row of vehicles at the school parking lot and begins shooting at the school, McCraw said. Patrol vehicles begin to arrive at the funeral home.

There was no school resource officer that confronted the suspect outside the school, as officials had previously described, McCraw said. A school resource officer was not on scene but heard the 911 call about a man with a gun, drove to the area and sped to the back of the school, to a person he thought was the suspect but was a teacher, McCraw said.

"In doing so, (the school resource officer) drove right by the suspect, who was hunkered down behind a vehicle, where he began shooting at the school," McCraw said. Multiple shots were fired by the suspect, he added.

FIRST OFFICERS ENTERED THE SCHOOL MORE THAN ONE HOUR BEFORE SHOOTER WAS KILLED

11:33 a.m.: The shooter enters the school and begins shooting into a classroom, which is connected to a second class. He shot "at least" 100 rounds, McCraw said.

11:35 a.m.: Three Uvalde Police Department officers enter through the same door as the suspect. Another three Uvalde police officers and a county sheriff follow, McCraw said, for a total of seven officers on scene.

The three initial officers went directly to the class door, which was closed, and two received grazing wounds from the shooter, McCraw said.

11:37 a.m.: Another 16 rounds are fired in the following minutes.

11:42 a.m.: A source close to a teacher receives a text saying there was an active shooter on campus. CNN saw the text chain and confirmed the timestamps.

11:43 a.m.: Robb Elementary announces on Facebook it's under a lockdown status "due to gunshots in the area," adding that "the students and staff are safe in the building."

Roughly 11:44 a.m.: Officers are calling for additional resources, equipment, body armor, negotiators and evacuating students and teachers, Escalon said Thursday.

11:51 a.m.: More officers arrive on scene, McCraw said.

12:03 p.m.: Officers continue to arrive in the hallway of the school. "There's as many as 19 officers at that time in that hallway," McCraw said.

12:03 p.m.: A young girl from inside one of the adjoining classrooms calls 911, identifies herself and whispers the classroom she is in. The call lasted a minute and 23 seconds. She calls back several minutes later and says multiple people are dead.

12:10 p.m.: First group of deputy US Marshals arrives on scene to assist "federal, state, and local law enforcement already on scene," the Marshals Service said in its statement.

12:13 p.m.: The girl calls 911 again, McCraw said.

12:15 p.m.: Members of the Border Patrol's tactical unit, BORTAC, arrive on scene, McCraw said.

(When Border Patrol agents began to arrive, the officer in charge of the situation had already made the determination that it was a barricaded subject situation, a source familiar with the situation said. The team then waited, not breaching the classroom where the shooter was holed up -- until nearly 40 minutes later.

McCraw said the person who made that decision was the school district police chief, calling it the "wrong decision," not to engage with the gunman sooner.)

12:17 p.m.: Robb Elementary announces on Facebook that there is an active shooter at the school and authorities are at the scene.

12:16 p.m.: The girl calls 911 again and tells dispatchers there are eight to nine students alive, McCraw said.

12:19 p.m.: Another person calls 911 from one of the two classrooms and hangs up when another student tells her to, McCraw said.

12:21 p.m.: The suspect fires again. He was believed to be at the door, McCraw said.

Law enforcement move down the hallway.

12:21 p.m.: Three shots fired are heard from another 911 call made.

12:36 p.m.: The initial student who called 911 calls again, is told to be very quiet and tells dispatchers "he shot the door,' McCraw said. The call lasted 21 seconds.

12:43 p.m.: The young girl asks dispatchers to "please send the police now."

12:47 p.m.: The student asks for police again, McCraw said. A minute earlier, she had said she could hear the police next door.

12:50 p.m.: Law enforcement breach the locked classroom door using keys from a janitor, McCraw said. They shoot and kill the suspect.

12:51 p.m.: Through the young girl's 911 call, there are loud noises and officers can be heard moving children out of the room, McCraw said. The child goes outside and the call cuts off.

The suspect purchased and had a total of 1,657 total rounds of ammunition, McCraw said -- at least 315 of them were inside the school.

And 142 of those were spent cartridges.