The mother of the 18-year-old gunman accused of killing 21 at an elementary school in Uvalde, Texas, this week told ABC News in an interview that her son was "not a monster," but that he could "be aggressive."

"I had an uneasy feeling sometimes, like 'what are you up to?" Adriana Reyes told ABC News' Chief National Correspondent Matt Gutman in an interview at her home. "He can be aggressive... If he really got mad."

Reyes' son, accused shooter Salvador Ramos, allegedly purchased two assault rifles in the days after he turned 18 and used them to carry out the second-deadliest school shooting in U.S. history -- all within a span of eight days, authorities said.

Twenty-one people, including 19 third- and fourth-grade children, were killed in the attack, law enforcement officials said. Two teachers were killed, as well. Another 17 people were wounded, including three law enforcement officers. Ramos' grandmother, who police say was the accused gunman's first victim, is hospitalized in stable condition.

"We all have a rage, that some people have it more than others," Reyes said.

Reyes expressed sympathy for the children who were killed and the parents who lost them multiple times during the interview, but said she was not aware that her son had been buying the weapons.

"Those kids… I have no words," Reyes said through tears. "I don't know what to say about those poor kids."

Some classmates told ABC News that Ramos was known for fighting and threatening fellow students. They said he exhibited increasingly disturbing behavior over the past two years, threatening at least one classmate and stalking others, and that he claimed to have cut scars into his face.

Ramos is accused of shooting his grandmother at their home in Uvalde before driving his grandparents' car to Robb Elementary School and opening fire.

"Anyone who shoots his grandmother in the face has to have evil in his heart," Texas Gov. Greg Abbott said during a press conference Wednesday. "But it is far more evil for someone to gun down little kids."