Sadie Rodriguez, the mother of Shana Fisher, 16, told the newspaper that her daughter rejected four months of aggressive advances from accused shooter Dimitrios Pagourtzis, 17, at the Santa Fe high school.
Fisher finally stood up to him and embarrassed him in class, the newspaper quoted her mother as writing in a private message to the Times.
"A week later he opens fire on everyone he didn't like," she said. "Shana being the first one."
Rodriguez could not independently be reached for comment.
If true, it would be the second school shooting in recent months driven by such rejection.
In March, a 17-year-old Maryland high school student used his father's gun to shoot and seriously wound a female student with whom he had been in a recently ended relationship, police said.
As the investigation enters its third day on Sunday, no official motive has been announced for the massacre, the fourth-deadliest mass shooting at a U.S. public school in modern history.
Classmates at Santa Fe High School, with some 1,460 students, described the accused shooter, as a quiet loner, who played on the school's football team. He wore a black trench coat to school in the Texas heat on Friday and opened fire with a pistol and shotgun.
He has provided authorities little information about the shootings, his attorney Nicholas Poehl said, adding: "Honestly because of his emotional state, I don't have a lot on that."
Texas' governor, Jim Abbott, a Republican, told reporters that Pagourtzis obtained firearms from his father, who had likely acquired them legally.
Abbott also said Pagourtzis wanted to commit suicide, citing the suspect's journals, but did not have the courage to do so.
Pagourtzis' family said in a statement they were "saddened and dismayed" by the shooting and "as shocked as anyone else" by the events. They said they are cooperating with authorities.
All schools in Santa Fe will be closed Monday and Tuesday, officials said.
Pagourtzis, who police said has confessed to the shooting, was being held without bond Sunday at a jail in Galveston.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
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