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A sense of “resentment” might have played a role in a 28-year-old’s deadly attack on the private Christian school they once attended, Nashville police said Monday.

The shooter, Nashville resident Audrey Hale, had no previous criminal record before opening fire at The Covenant School, killing three children and three adults, authorities said.

Hale had carefully planned the attack with detailed maps and surveillance, police said.

While the shooter might have targeted The Covenant School, Drake stopped short of saying Hale was going after any specific people. There were also indications that Hale had planned to target other locations, Drake told NBC News.

Covenant’s head of school, Katherine Koonce, 60, was among those killed.

“She targeted random students in the school ... whoever she came in contact with, she fired rounds,” Drake said, referring to the shooter.

Hale shot open a locked door to gain entrance to the school, officials said.

The former student is alleged to have left behind writings being studied by local and federal investigators.

“We have some writings that we’re going over that pertain to this date, the actual incident,” Drake told reporters hours after the shooting. “We have a map drawn out of how this was all going to take place.”

He said Hale was transgender.

Officials “feel that she identifies as trans, but we’re still in the initial investigation into all of that and if it actually played a role into this incident,” Drake said.

Former school headmaster Bill Campbell said that he remembered Hale as a third-grader in 2005 and fourth-grader in 2006, citing yearbooks he keeps.

“It’s just an absolute tragedy what’s happened by the former student and what she did to the teachers and students,” Campbell said.

Hale was not listed as a student in the annuals in the fifth or sixth grade, so Campbell said he believes Hale transferred after the fourth grade.

“I’ve looked back in my annuals and I do remember her as a former student,” the former headmaster said. “She was just one of our young ladies. ... She was just a typical co-ed. A typical student.”

Campbell could not recall any issues Hale might have had at the time that could have raised red flags.

"I think about this student and our relationship at the time she was there, there was nothing extraordinary and unusual," the former school administrator said. "She was loved and appreciated like all of our students.”

Hale came to the school on Monday with two AR-style weapons and a handgun, Drake said.

Two of those three weapons were legally obtained in the Nashville area, he added.

Hale was confronted by five officers and two of them opened fire, killing the attacker, authorities said.

A car near the scene helped police determine who the shooter was, and investigators were combing through Hale’s home, he said.

Police tactical units were spotted using a grenade-like device to get inside the residence, which is listed as Hale's home.

"This is a great family, and it's a tragedy," a neighbor said.

There were no answers Monday afternoon at multiple phone numbers listed for Hale’s mother and brother.

Drake praised his officers for taking on the shooter immediately.

“It could have been far, far worse,” he said.

Hale's writings also indicated that The Covenant School wasn't the killer's only target, authorities said.

"It indicates that there was going to be shootings at multiple locations, and the school was one of them," Drake told NBC News.

Officers prevented more deaths thanks to their fast work, Nashville Mayor John Cooper said.

"Let us praise our first responders, 14 minutes, 14 minutes, I believe under fire, running to gunfire," Cooper told reporters.