What is known about the Valentine's Day slayings at a South Florida high school suggests the carnage was planned with chilling precision: The alleged shooter - armed with an assault-style weapon - pulled a fire alarm and waited as his victims poured into the halls.
What remained to be unravelled Thursday (Eastern Time) was what drove the teenage suspect, Nikolas Cruz, to bring his rage to a school he once attended and claim the lives of students he once called classmates - in what would become the nation's second deadliest school shooting with a toll of at least 17 lives.
Early Thursday, Cruz was booked on 17 counts of "murder premeditated".
Investigators now were left to piecing together the narrative behind the massacre even as political leaders and a grieving community once again grapple with questions over gun control and how to better protect campuses, churches and other sites from becoming the next targets.
"We've got the people prepared, we have prepared the campuses, but sometimes people still find a way to let these horrific things happen," said Donna Korn, a board member of Marjory Stoneman Douglas High School in Parkland, a tidy suburb on the edge of the Everglades northwest of Fort Lauderdale.
From former acquaintances at the school, a portrait emerged of Cruz as an increasingly erratic and troubled soul before he was expelled last year.
He "started progressively getting a little more weird," said 17-year-old Dakota Mutchler. Cruz, he said, was selling knives out of a lunchbox, posting on Instagram about guns and killing animals, and eventually "going after one of my friends, threatening her."
"When someone is expelled," Mutchler told The Washington Post, "you don't really expect them to come back. But, of course, he came back."
When he did, police said, Cruz was outfitted for a siege. Cruz had a gas mask, smoke grenades, " a load of ammunition and an AR-15 rifle. Besides the dead, at least 15 others were wounded before Cruz attempted to slip away amid the panicked students.
"It's a horrific, horrific day," said Broward County Sheriff Scott Israel, whose own triplets graduated from the high school. "It's catastrophic. There really are no words." The victims included several students and adults, authorities said.
"It is a day you pray every day you don't have to see," said Broward County Public Schools Superintendent Robert Runcie, reflecting on one of the nation's deadliest school shootings. The shooter came to the school armed with weaponry that evoked a battlefield, not a school located down the street from an equestrian park. He carried "countless magazines" and an AR-15 rifle, Israel said. It was unclear if the shooter had a second weapon, the sheriff said.
Just after 3pm Wednesday, Michael Nembhard - a retiree who lives in Coral Springs, which sits just south of Parkland - was sitting in his garage watching the TV news when he heard an officer yell, "Get on the ground!"
He said he looked out and saw police arrest the suspect in the school shooting. The teenager was on the ground, wearing a burgundy hoodie and dark pants.
"The cop had his gun drawn and pointed at him," Nembhard said in a phone interview. "The kid's face was turned away, so I couldn't see anything."
Jim Gard, a math teacher at Stoneman Douglas High School, which is named for an icon of the South Florida environmental movement - said he taught the shooting suspect last year.
"I had him almost all year. He just looked like a regular high school kid. Nothing outstanding. He didn't act up in class, wasn't loud or boisterous," Gard said.
But at some point during the school year, Gard said, the school administration sent out a note with a vague suggestion of alarm, asking teachers to keep an eye on Cruz. "I don't recall the exact message, but it was an email notice they sent out."
Within hours of Cruz's arrest, authorities began to pore over his social media postings. Some "are very, very disturbing," said Israel, the Broward Country sheriff.
An Instagram account that appeared to belong to the suspect showed several photos of guns. One appeared to show a gun's holographic laser sight pointed at a neighborhood street. A second showed at least six rifles and handguns laid out on a bed with the caption "arsenal." Other pictures showed a box of large-caliber rounds with the caption "cost me $30." One of the most disturbing appeared to show a dead frog's bloodied corpse.
Cruz and a half brother were adopted as babies by Lynda and Roger Cruz, according to a relative in New York. Roger died years ago and Lynda died last fall, the relative said. Around Thanksgiving, Nikolas Cruz moved in with the family of a friend from Stoneman Douglas High School, said Jim Lewis, an attorney representing the family.
"The family brought him into their home. They got him a job at the local dollar store. They didn't see anything that would suggest any violence," said Lewis, who declined to identify them. "He was depressed, maybe a little quirky. But they never saw anything violent."
Lewis said Cruz already owned the AR-15 rifle when he moved in with the family. "It was his gun. . . . It was secured in a gun cabinet in the house, but he had the key to it," Lewis said.
Cruz was enrolled in a programme to obtain a GED (General Educational Development, equivalent to a high school diploma) Lewis said. But on Wednesday, he didn't attend the family, telling the family something to the effect of "I don't go to school on Valentines day," Lewis said.
After the shooting, Ryan Gutierrez, 18, a senior, walked the 3km from the school to a 7-Eleven in Coral Springs - the nearest spot where his parents could meet him. Police cars blocked every other road leading to the school.
His parents had already been reunited with his sister, Nicole, a freshman at the school.
As Gutierrez approached, his mom ran up, hugged him hard and started crying. Gutierrez held her tight, comforting her. His father came up and hugged them both.
"This has been so horrible, the most horrible day anyone can imagine," Gutierrez's mother, Diana Gutierrez, said, trying to stop her tears. "It's unreal, just unreal. I still don't believe it. You don't think it will ever happen to you and your children."
This is at least the third school shooting this year, and one of the deadliest on record. Beginning with Columbine 19 years ago, more than 150,000 students attending at least 170 primary or secondary schools have experienced a shooting on campus, according to a Washington Post analysis of online archives, state and federal enrollment figures, and news stories. That doesn't count dozens of suicides, accidents and after-school assaults that have also exposed children to gunfire.
It was also the second-deadliest at a US public school after the 2012 massacre of 20 first-graders and six staff members at Sandy Hook Elementary in Newtown, Conn.
It is likely to revive a debate over gun control, though efforts to legislate restrictions on firearms following previous school shootings largely proved fruitless.
US President Donald Trump said he had been briefed on the shooting and tweeted, "My prayers and condolences to the families of the victims of the terrible Florida shooting. No child, teacher or anyone else should ever feel unsafe in an American school."
Denise Loughran was reunited with her 17-year-old son, Liam, within a few hours of the shooting. But they still hadn't heard from his sister, Cara, a freshman.
"Her phone must be in her backpack, and they made them drop their backpacks when they ran out," Loughran said. "This has just been chaos. I couldn't get near the school. My husband took a bike to try to get there, and they ended up sending him to the hotel where they said they were taking the kids.
"But she's not there."