These are the security measures the Uvalde School District had in place

The Texas school district where 19 children elementary school children and two adults were killed appeared to have security measures in place in the event of a school shooting, records show.

Included in the Uvalde Consolidated Independent School District preventative security measures plan were lockdown protocols, a system in place to report threats and bullying, threat assessment teams, as well as the monitoring of social media, according to school district documents featured on the school district's website.

The school district also has its own police force with four officers and partners with local law enforcement, according to the document. Secondary campuses have staff who patrol door entrances, parking lots and perimeters of campuses.

The standard response plan to reunify parents with students after an incident was provided in a handout for parents with pickup instructions as well as a "Reunification Information" form for parents to complete that identifies the student and the person who is to pick them up, the documents show.

Other preventative measures include motion detectors and alarm systems, a classroom door policy that requires keeping doors locked at all times, and staff training for emergency protocols. In addition, case managers, social workers and licensed professional counselors are on hand to support students and families, according to the documents.

The suspect, 18-year-old Uvalde High School student Salvador Ramos, allegedly shot his grandmother at a separate scene before crashing a car outside of the Robb Elementary School and emerging from the vehicle with an AR-15-style rifle, multiple law enforcement sources told ABC News. She was airlifted to the hospital and is in critical condition, authorities said

As Ramos approached, the school, he was engaged by a school district police officer, who was then allegedly shot by Ramos, sources said.

At one point, students heard banging on a window before their teacher saw the shooter with a "big gun," a fourth-grade student who was inside the school at the time said in an interview with ABC News, describing the "nonstop" gunshots that followed.

Ramos then entered the school, barricaded himself in a classroom and opened fire, killing 19 students and a teacher, law enforcement sources said.

There, Ramos was shot and killed after trading gunfire with an Uvalde ISD officer and Border Patrol Tactical Unit agents, sources said. Investigators are going through ballistics to determine who fired the shot that killed the suspect.

Authorities have not detailed how Ramos was able to enter the school and classroom despite the security measures in place. Ramos entered the school through a back door, officials said during a press conference Tuesday.

Every door around the building of the school should be locked, and visitors should only be able to access the building from a single point of entry through a secure vestibule that remains locked until they are authorized, security expert Paul Timm, vice president of Facility Engineering Associates, told ABC Chicago station WLS.

Timm said that all classroom doors should remain locked as well, but added that schools often resist that measure because it gets in the way of operations, such as students needing to use the restroom.

Two responding police officers were also injured, Texas Gov. Greg Abbott said in a press conference on Tuesday.

The students killed were mainly third and fourth graders, according to law enforcement sources.

The father of a 10-year-old Jackie Cazares, who was killed in the shooting, blamed the school district for not having locked doors and lacking training and protocol and police for not responding fast enough.

"There was at least 40 lawmen armed to the teeth, but didn't do a darn thing till it was far too late," Jacinto Cazares told ABC News in a statement.

A 2017 to 2020 school district audit by the Texas School Safety Center found that only 200 of the districts reviewed had viable active shooter plans. Of the remaining 822 districts, 626 districts in the state did not have a policy in place, and 196 districts had an insufficient policy, the audit found.

The report did not list specific school districts in Texas and what they ranked.

"Significant appropriations" are provided to ensure that the local school systems around the state have resources for the school safety allotment under Texas Senate Bill 11, Mike Morath, commissioner of education for the Texas Education Agency, said during Tuesday's press conference.

The Texas School Safety Center and the Texas Education Agency provide technical assistance and training to school district leaders to ensure that the threat assessment protocols and these procedures, including threat response plans of the operational multi hazard plans are practiced on recurring basis, Morath said.

"There have been, essentially, fairly significant efforts to bolster those managerial practices and these detective or preventative practices in schools all over the state of Texas," Morath said. "And we will continue to do more."

The shooting in Uvalde, a small town about 90 minutes west of San Antonio, comes just days after a deadly mass shooting in Buffalo, New York and amid a rapid rise in active shooter incidents in the country.

Ramos legally purchased two AR-style rifles just days after his 18th birthday, law enforcement officials said.

ABC News' Jack Date and Meredith Deliso contributed to this report.