The 18-year-old suspect, a student at Uvalde High School, is also dead, Gov. Greg Abbott said.
Abbott said the suspect "shot and killed horrifically and incomprehensibly" more than a dozen students and a teacher.
The incident began when the suspect, identified by Abbott and law enforcement sources as Salvador Ramos, shot his grandmother. Initial reports from law enforcement said the shooter's grandmother had died, but as of 11 p.m., law enforcement sources told ABC News the grandmother was in critical condition but still alive.
Texas public safety officials said Ramos then left his home in a truck, which he crashed in a ditch outside Robb Elementary School. He got out with an AR-15 style rifle and numerous magazines, law enforcement sources told ABC News. He was also wearing body armor.
As he approached the school, an Uvalde ISD school resource officer engaged the suspect, but Ramos shot him, law enforcement sources said.
The shooting took place as soon as the suspect entered the school and all of the victims at the school died "in the same location inside the school," authorities told ABC News.
Inside the school, the suspect traded fire with Uvalde ISD officers and Border Patrol Tactical Unit agents, sources told ABC News.
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"U.S. Border Patrol Agents responded to a law enforcement request for assistance re an active shooter situation inside Robb Elementary School in Uvalde. Upon entering the building, Agents & other law enforcement officers faced gun fire from the subject, who was barricaded inside," a DHS spokeswoman said in a statement posted to Twitter.
Law enforcement sources told ABC News that they are now tracing the AR-15 style rife the shooter used, and also recovered the body armor and numerous magazines of ammunition.
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ABC News confirmed that fourth-grade teacher Eva Mireles was among those killed. Mireles worked in the Uvalde Consolidated Independent School District for 17 years, her aunt, Lydia Martinez Delgado, said in a statement.
"This is my hometown, a small community of less then 20,000. I never imagined this would happen to especially to loved ones," Delgado said.
Family also identified one of the children killed was 10-year-old Xavier Lopez. He was in fourth grade. His cousin told ABC News his mother had attended his awards ceremony just a couple hours before the shooting, not knowing it would be the last time she saw her son.
"When parents drop their kids off at school, they have every expectation to know that they're going to be able to pick their child up when that school day ends. And there are families who are in mourning right now," Abbott said. "The state of Texas is in mourning with them for the reality that these parents are not going to be able to pick up their children."
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Two responding police officers were among those injured, Abbott said. They are expected to survive, he said.
The shooter's social media is being combed by investigators, and he reportedly sent videos and photos of guns to other users on multiple platforms, ABC News reports.
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Uvalde Memorial Hospital had said 15 students were being treated in the hospital's emergency department in the wake of the incident. Two patients were transferred to San Antonio for treatment, while a third was pending transfer, the hospital said. A 45-year-old was also hospitalized after getting grazed by a bullet, the hospital said.
University Health in San Antonio said it had four patients from the shooting incident -- a 66-year-old woman and a 10-year-old girl who were both in critical condition, a 10-year-old girl in good condition and a 9-year-old girl in fair condition.
Two adult victims of the school shooting are being treated at Brooke Army Medical Center in San Antonio, according to an Army official, who did not have an update on their conditions.
ABC News special report: 14 students, 1 teacher dead after shooting at Texas elementary school
A number of the shooting victims are children of Customs and Border Patrol agents, law enforcement sources told ABC News.
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Uvalde Mayor Don McLaughlin did not confirm casualties, but told ABC News in a text message that "this is a very bad situation." He said the office is trying to contact parents before releasing any information.
The shooting occurred shortly after 11:30 a.m. local time, police said.
The school, which has students in the second, third and fourth grades, informed parents shortly after 2 p.m. that students had been transported to the Sgt. Willie Deleon Civic Center, the reunification site, and could be picked up.
Parent Ryan Ramirez told San Antonio ABC affiliate KSAT he had gone to the civic center and the elementary school trying to find his fourth grade daughter in the wake of the shooting.
"[I'm] just confused and worried. I'm trying to find out where my baby's at," he told the station.
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School has been canceled for the remainder of the school year, including all extracurriculars on Wednesday and Thursday. The graduation ceremony "will be addressed at a later time," officials said. Grief counseling will also be made available, according to officials.
Uvalde, Texas, is located about 90 minutes west of San Antonio.
The Bexar County Sheriff's Office and San Antonio Police Department are sending aid, and the FBI is responding.
The Houston Field Division of the Bureau of Alcohol, Tobacco, Firearms and Explosives also said it is assisting in the investigation.
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Department of Homeland Security Secretary Alejandro Mayorkas has been briefed on the situation and the agency "is actively coordinating with federal, state, and local partners," a spokesperson said. Customs and Border Protection officials in the area also responded to the scene.
The National Counterterrorism Operations Center believes there is "no known terrorism nexus" at this time, according to a law enforcement bulletin obtained by ABC News.
“Why are you here?!"— ABC News (@ABC) May 24, 2022
A furious Sen. Chris Murphy demands answers from senators following Texas school shooting.
“Why do you spend all this time running for the United States Senate...if your answer, is as the slaughter increases, as our kids run for their lives—we do nothing?” pic.twitter.com/9fkJ13vWGd
ABC News' Pierre Thomas, Luke Barr, Aaron Katersky, Nicholas Kerr and Mireya Villarreal contributed to this report.
This is a developing story. Check back for updates.