CNN — The woman standing outside Nashville’s Covenant School was calm and direct. The building was locked down, but two young students were still missing.

“Upstairs,” she told Officer Rex Engelbert, “there are a bunch of kids.”

After being let in the school by another staffer, Engelbert, whose body camera footage provided a grueling blow-by-blow of the final throes of a massacre that left three 9-year-old children and three staff members dead, rushed inside.

Minutes later, the 28-year-old alleged killer was dead – shot by police on the second floor of the private Christian elementary school.

The extraordinary videos, which show the officers racing down hallways lined with backpacks and children’s jackets before their confrontation with the suspect, have horrified a nation once again grasping for answers in the wake of another mass shooting.

New details about the suspected killer, Audrey Hale, continue to trickle out, and the search for a motive continues.

Hale was a former student and had been under care for an “emotional disorder,” Metro Nashville Police Department Chief John Drake said Tuesday. Law enforcement also revealed that Hale had left behind writings suggesting the attack had been mapped out beforehand in devastating detail.

An ominous message

That Monday morning began with an unexpected message.

Averianna Patton, a Nashville radio host and Hale’s former grade school basketball teammate, received a series of messages sent by Hale, she told CNN.

“I’m planning to die today,” Hale wrote, adding that Patton would hear about it in the news.

Patton had not been in contact with Hale for years until she received that direct message on Instagram at 9:57 a.m., she said.

“One day this will make more sense,” Hale added in the messages. “I’ve left more than enough evidence behind. But something bad is about to happen.”

Unnerved by the cryptic messages, Patton said she contacted her father, who urged her to act.

Patton contacted a suicide prevention line and then the Nashville Davidson County Sheriff’s Office.

She called the police at 10:13 a.m., she said – the same time the department received its first 911 call about an active shooter at the school.

She waited seven minutes to speak with a dispatcher. By then, the deadly rampage at Covenant School had already started.

A school turned war zone

Standing in front of a side entrance to the school, Hale fired off shots that shattered the glass door, surveillance video shows. Armed with three weapons, Hale can be seen ducking through the broken door and entering.

Custodian Mike Hill was shot through that door, police said. The 61-year-old, known to the school’s students as “Big Mike,” was among the first killed.

In the minutes that followed, three children were killed – Evelyn Dieckhaus, William Kinney and Hallie Scruggs – and two more adults – substitute teacher Cynthia Peak, 61, and Katherine Koonce, 60, the head of the school.

At 10:18 a.m., the shooter is seen on surveillance footage walking through the hallways as fire alarm lights go off, pointing an assault-style weapon.

Six minutes later police entered to the sound of blaring alarms. Engelbert’s body camera footage shows the officers hurrying through halls decorated with children’s artwork, checking classrooms in an increasingly desperate search for the shooter. Some doors open to darkened rooms, small desks and chairs empty; others are locked.

Then, the sound of gunfire rattles the hallway. “I think it’s upstairs, it sounds like it’s upstairs,” an officer says in the video. As police run up the stairwell towards the heavy booms, it grows louder.

They encountered the shooter in a second-floor lobby with high, bright windows that look down on the parking lot.

At 10:27 a.m., Engelbert fired four rounds at the shooter, who is seen collapsing in his body camera footage.

Fellow officer Michael Collazo then stepped forward to the fallen shooter with a handgun, letting off more rounds, yelling, “Stop moving!”

“Suspect down!” Collazo screamed twice into his radio. Then, after a labored breath, he sent an order to “Hold the air” – a request to stop all unnecessary radio traffic so his team can communicate across clear airwaves.

‘Our hearts are completely broken’

The killings marked the 19th deadly shooting at a US school or university this year alone and come less than a year after the attack in Uvalde, Texas, where 19 children and two teachers were murdered by a man armed with AR15-style rifles.

In the aftermath, community members paid their respects to the lives lost, placing bouquets, toys, balloons and a flower-wreathed cross at a makeshift memorial just outside the school.

“Our hearts are completely broken,” Evelyn’s family said in a statement. “We cannot believe this has happened. Evelyn was a shining light in this world.”

Hallie was the daughter of Covenant Presbyterian Church Lead Pastor Chad Scruggs, according to a statement by Park Cities Presbyterian Church in Dallas, a sister church Scruggs formerly served.

“We love the Scruggs family and mourn with them over their precious daughter Hallie,” the Texas congregation’s Senior Pastor Mark Davis said. “Together, we trust in the power of Christ to draw near and give us the comfort and hope we desperately need.”

Hill, Peak and Koonce were also mourned by their loved ones.

Republican Tennessee Gov. Bill Lee, whose wife, Maria, had been close friends with Peak, said: “Maria woke up this morning without one of her best friends, Cindy Peak…Cindy was supposed to come over to have dinner with Maria last night after she filled in as a substitute teacher yesterday.”

Lee’s wife, Peak and Koonce, had all taught together and known one another “for decades,” Lee added.

Koonce had been a member of the faculty for 16 years, according to a statement on the school’s website, and is survived by her husband and two children, both of whom graduated from Covenant.

“The faculty, staff, and students who worked with her mourn the loss of a dedicated educator, a tenacious leader, and a dear friend,” the statement said.

Koonce’s friend and former colleague Jim Lee told CNN he and his wife flew to Nashville after learning she had been killed. They described a live and witty character, deeply connected to her students and staff, but also a fierce administrator.

“She had this amazing confidence, but she was a person of grace,” Lee said. “She was an educator, but she also had great pastoral and counseling and nurturing skills – or she had those CEO skills that could tell you that you need to kind of get in your place.”

On Facebook, Hill’s daughter Brittany Hill wrote that her dad “absolutely loved” his work at the school.

“I have watched school shootings happen over the years and never thought I would lose a loved one over a person trying to solve a temporary problem with a permanent solution,” she said.

“I am so sorry for the loss of those children.”