Nineteen students and two teachers at Robb Elementary School in Uvalde were killed in the mass shooting on Tuesday.
The massacre, carried out by 18-year-old Salvador Ramos, was the deadliest school shooting in the US since Newtown, Connecticut, nearly a decade ago.
Speaking after her son was shot dead by law enforcement officers, Adriana Martinez Reyes said she had no explanation for his attack on the school.
“I have no words to say. I don’t know what he was thinking,” a distraught Ms Reyes told CNN affiliate Televisa, when questioned by a reporter from the inside of a vehicle.
“He had his reasons for doing what he did and please don’t judge him. I only want the innocent children who died to forgive me.”
Asked to describe her son, she said: “He was very quiet ... he didn’t bother anybody.”
Ramos’s father, a 42-year-old also named Salvador Ramos, expressed remorse for his son’s actions on Thursday.
“I just want the people to know I’m sorry, man, [for] what my son did,” the elder Ramos told the Daily Beast. “He should’ve just killed me, you know, instead of doing something like that to someone.”
The 18-year-old arrived at the school having stolen the family truck and shot his grandmother in the face. She called the police on him herself, and remains in a critical condition in hospital.
A high school dropout, Ramos had no criminal record and no history of mental illness, and investigators are still seeking a motive.
He had previously hinted on social media that an attack could be coming, and according to Texas governor Greg Abbott, he had written an online message saying he was about to “shoot up an elementary school” minutes prior to the attack.
Mr Abbott, who had earlier described Ramos as “the sheer face of evil”, said on Friday that he was “livid” about what he had learned about the police response, as it became apparent that law enforcement failures may have contributed to the death toll.
“My expectation is that the law enforcement leaders that are leading the investigations ... they get to the bottom of every fact with absolute certainty,” Mr Abbott said. “There are people who deserve answers the most.”
He added: “Law enforcement is going to earn the trust of the public by making sure they thoroughly and exhaustively investigate exactly what happened,”
Additional reporting by AP