Sadie Rodriguez said her daughter, Shana Fisher, had made clear that she was not interested in Dimitrios Pagourtzis.
"He continued to get more aggressive," Rodriguez told The Associated Press in an interview conducted Saturday via Facebook. "She finally stood up to him and embarrassed him."
The incident took place one week before the shooting, Rodriguez said. Police have not yet said what might have motivated the attack.
Asked about Rodriguez's allegation, a lawyer for the Pagourtzis's family said he hadn't heard about any such interaction between Pagourtzis and any of the victims and therefore couldn't comment.
"That's news to me," said lawyer Nicholas Poehl, though he cautioned that he'd spent much of the day disputing false rumors about the teen's personal life. Earlier Saturday, Poehl issued a statement from the family saying that they were "as shocked and confused as anyone else by these events that occurred" and asked for privacy.
"While we remain mostly in the dark about the specifics of yesterday's tragedy, what we have learned from media reports seems incompatible with the boy we love," the family's statement said.
Representatives of the school district did not immediately respond to questions about Rodriguez's comments.
A junior at Santa Fe High, Pagourtzis hid a shotgun and a handgun under his trenchcoat before opening fire in a first-period art class on Friday, according to an affidavit filed by police. The FBI announced Saturday that it had taken the lead in the joint investigation, at the request of local authorities.
"He gave a statement admitting to shooting multiple people inside the Santa Fe High School with the intent on killing people," the police affidavit said. "Dimitrios advised he did not shoot students he did like so he could have his story told." The attack ended when Pagourtzis walked out of a classroom and surrendered to police.
Authorities say Pagourtzis planned the killings, carried out with weapons owned by his father. Though Pagourtzis allegedly wrote about his intention to carry out the attack, authorities have not indicated a motive for the violence.
Gov. Greg Abbott said Pagourtzis had planned to kill himself after the shooting, but that Pagourtzis told police "that he didn't have the courage to commit the suicide."
Some Santa Fe High students indicated that Pagourtzis may have had one or more targets, though details were not clear. Breanna Quintanilla, 17, a junior, said she was in her art classroom Friday morning in what she described as a "perfectly normal day" when she heard gun shots.
Quintanilla said when Pagourtzis first walked in, he pointed at one person and said, "I'm going to kill you." Quintanilla, who was wounded escaping the classroom, did not identify that student.
At a news conference, Abbott said Pagourtzis had not previously been in trouble. "The red-flag warnings were either nonexistent or very imperceptible," he said.
In the absence of a clear motive, attention turned to a hodge-podge of incendiary posts by Pagourtzis on social media, which were quickly removed when his identity first emerged. One Facebook post was a picture of a T-shirt reading "Born to Kill." Another showed a trench coat adorned with various pins associated with conflicting ideologies and Cthulhu, a mythical creature drawn from the work of horror fiction writer H.P. Lovecraft.
In the wake of the shooting, various reports that Pagourtzis was bullied have surfaced from his classmates. However, two Santa Fe High students who said they knew Pagourtzis said they had no reason to suspect that he was planning Friday's attack.
Student Michael Farina said he had known the alleged gunman since fifth grade and considered him a friend. He recalled Pagourtzis talked about buying a gun "when I grow up," but never discussed his dad's guns or anything about his family.
Farina said he and Pagourtzis also talked a lot about video games they liked, including so-called first-person shooter games. They'd talked about one, "Escape from Tarkov," as recently as lunch on the day before the shooting, he said.
Farina did not recall Pagourtzis ever being notably angry.
"He seemed relatively happy for the most part," he said.
Horwitz reported from Washington and Zimmerman from Springfield, Illinois. Associated Press writers Eric Tucker in Washington, Paul J. Weber in Austin, Texas, and John Mone and Nomaan Merchant in Santa Fe, Texas, contributed.
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