On the morning shooter Dylan Butler opened fire, a TikTok post believed to be from the shooter shows him inside a school bathroom posing with a blue duffel bag, captioned: "Now we wait."
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The song "Stray Bullet," by the German band KMFDM, accompanies the post - which has been removed from the platform. The band's lyrics have also been cited by the student gunmen who carried out the 1999 massacre at Columbine High School in Colorado, where 13 victims were killed, CNN has reported.
The gunman in Iowa "made a number of social media posts in and around the time of the shooting" on Thursday morning, said Mitch Mortvedt, assistant director of the Iowa Department of Public Safety Division of Criminal Investigation. "Law enforcement are working to secure those pieces of evidence," he added.
New details also are emerging about whether Butler may have been bullied, though his motive might never be confirmed because the gunman is dead. He was found with a self-inflicted gunshot wound, Mortvedt said.
Still, investigators are trying to determine why Butler entered the school with a pump-action shotgun and small-caliber handgun, Mortvedt said. Officers also discovered a rudimentary explosive device after the killer went on the deadly rampage before classes started on the first day back to school since winter break.
The attack marked the second shooting on a US school property in just the first few days of this year. Last year, more than 80 such shootings erupted on school grounds nationwide, a CNN analysis shows.
So far this year, the US this also has suffered an average of at least one mass shooting every day in which at least four victims are shot, according to the Gun Violence Archive.
Now, survivors and witnesses in a small city near Des Moines are echoing the same sentiment as others who have suffered the terror of mass shootings in America:
"It doesn't feel real," student Rachael Kares told CNN. "This is like one of those things where you see on TV and you're like that never gonna linger its way toward my community, but it does happen. It's really real."
'Everything just went to chaos'
As students dug into cafeteria breakfasts and wrapped up band practice Thursday morning, one of their peers stepped into the building with two guns and began firing - sending kids and teachers scrambling for safety.
The shooting happened before the classes at Perry High School, which shares a campus with Perry Middle School. "It's our understanding that there was a breakfast program going on, so there may have been students of different grades ... in the school at that time," Mortvedt said.
At first, the gunshots were mistaken by some as disruptive noise - a popped balloon, a dropped bag - two witnesses said.
Then the horror of reality set in.
"The whole cafeteria went silent," high school student Angie Orellana told CNN. "Then more shots continued, and everything just went into chaos. I just saw the principal start running and all my friends, and I just got out of there."
Nearby, Kares was wrapping up jazz band practice when she heard four shots resonate through the hall. A burning smell lingered in the air. Then another shot came.
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"Our band teacher looked at us, and he just goes, 'Run!'" she told CNN's Anderson Cooper. "None of us hesitated. We just all got up and ran."
Kares and her friends ran outside, trying to get "anywhere away from school," she said. "We just kept going."
Perry High School Principal Dan Marburger tried to save students by talking to and distracting the shooter during the deadly attack, his daughter Claire Marburger posted on social media.
"As I heard of a gunman, I instantly had a feeling my Dad would be a victim as he would put himself in harm's way for the benefit of the kids and his staff," she wrote. "It is absolutely zero surprise to hear he tried to approach and talk Dylan down and distract him long enough for some students to get out of the cafeteria."
The principal was wounded in the shooting and underwent surgery. He is stable, his daughter said.
He remains in critical condition after being shot multiple times, officials said.
Two other staff members were also injured.
More than 150 federal, state and local officers arrived to find students and staff sheltering in place or running from the building, Mortvedt said.
Inside, officers discovered the wounded gunman, as well as the explosive device, which authorities rendered safe, Mortvedt said.
The sixth grader who was killed was identified Friday as 11-year-old Ahmir Jolliff. He was shot three times.
Four other students were injured, Mortvedt said.
Two students remain hospitalized Friday. The rest have been released.
Officials previously said only five were injured.
All Perry district schools will be closed Friday, and counseling will be available.
The gunman's friends describe bullying
While the motive in the attack remains unclear, two teens described as friends of Butler said he had been bullied since elementary school, they told ABC News.
"He got tired, he got tired of the bullying, he got tired of the harassment," one friend told ABC News.
"We tried to be there when he needed us," another friend tearfully told ABC News. "Clearly, we weren't there for him enough."
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CNN is reaching out to authorities about whether the bullying claims have turned up in their investigation.
Stunned community searches for healing
Mourners grieved the loss of the slain sixth grader, who was "the sweetest boy - the one you want your kids to be friends with," Perry resident Jessica Conrad told CNN.
The principal was identified as one of the wounded in a statement from the Easton Valley Community School District in eastern Iowa, where Marburger graduated. The district learned of his injury through "family connections we have in our area," it said. Marburger has worked at schools in Perry for at least 25 years, according to the Perry Community School District's directory.
"This senseless tragedy has shaken our entire state to the core," Iowa Gov. Kim Reynolds said Thursday. "I want you to know that we'll work tirelessly to get the answers so that we can prevent it from happening again."
Mourners gathered for a vigil at a local park Thursday night, clutching candles as speakers shared messages of strength, sympathy and unity.
"Even though we are in a tiny town, the whole world is wrapping its arms around us tonight," said former Perry school student Andrea Niemeyer, who noted she's received condolences from as far as Washington state.
"We'll get through this because we have each other," she added.
Some expressed shock and disbelief, including a mother who said her husband tried to drop their children off early at the school but was turned away.
"I just found all throughout the morning as the time progressed that I would kind of lose my breath as I realized how lucky that was," Mindy Farmer said.
Later in the day, the question, "What now?" began to weigh on her, Farmer said. "I think you can start to feel kind of powerless."
Several vigil attendees offered up prayers, including local pastor Kathy Benton, who prayed for the students who "are going to be dealing with the memories and the sounds and everything."
"We pray for healing of their minds," she said. "We pray for healing of their emotions."
CNN's Caroll Alvarado, Andy Rose, Hannah Rabinowitz, Amy Simonson, Evan Perez, Aaron Pellish, Sharif Paget, Dakin Andone and Christina Maxouris contributed to this report.
ABC News contributed to this report.
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