In an interview with police, Nikolas Cruz said "he was the gunman who entered the school campus armed with an AR-15 and began shooting students that he saw in the hallway and on the school grounds," the document states.
"Cruz stated that he brought additional loaded magazines to the school campus and kept them hidden in a backpack until he got on campus to begin his assault."
Cruz has been charged with 17 counts of premeditated murder in the deaths of students and staff at Marjory Stoneman Douglas High School in Parkland on Wednesday afternoon.
Cruz is on suicide watch, his attorney told reporters Thursday. Public defender Gordon Weekes called his 19-year-old client a "deeply disturbed, emotionally broken" young man who has struggled with significant mental illness and trauma his entire life.
"He's gone through a lot in a very short period of time and that does not minimize the loss of those families, but we have to put that into the proper light," Weekes said, choking back tears in a crowd of reporters.
"He has some very difficult decisions to make shortly and we're going to assist him with those decisions."
Reports surface of possible warnings
The FBI received a report that the former student may have posed a threat before the shooting, a law enforcement source told CNN on condition of anonymity.
Authorities said Cruz, after Wednesday's slaughter at his former school, blended in with the students and staff evacuating the school but was arrested in a neighboring community later in the day.
In a brief Broward County court appearance Thursday by video from jail, he said nothing except to confirm his name to a magistrate judge, who ordered that he be held without bond. Wearing an orange jail jumpsuit, he nodded when he was told he couldn't post bail.
As for his online presence, one report to the FBI, the law enforcement source said, involved someone using Cruz's name commenting on a YouTube video claiming a desire to be a professional shooter.
YouTube video blogger Ben Bennight told CNN he warned the FBI in September about the YouTube message. "Im going to be a professional school shooter," read the comment, left by a user with the name Nikolas Cruz.
Rob Lasky, special agent in charge of the FBI's Miami division, acknowledged the report during a news conference in Parkland on Thursday, and said the FBI did a database review, but couldn't identify the person who wrote the comment.
The FBI didn't share the information with local authorities, the law enforcement source told CNN.
Meanwhile, officials have been alluding to mental health issues when speaking about Cruz publicly, but not offering details. Broward County Mayor Beam Furr, for example, told CNN Wednesday that the teen "had been dealing with mental health issues." CNN could not confirm the mayor's statements.
And Florida Gov. Rick Scott said Thursday he will have conversations with state leaders next week about "how to make sure individuals with mental illness do not touch a gun."
"If someone is mentally ill, they should not have access to a gun," Scott said.
A lawyer for the family Cruz was living with said they noticed he was depressed but they had no inkling that anything more serious may have been going on.
Cruz stands accused of committing one of the 10 deadliest mass shootings in modern US history.
At least 14 injured people were taken to area hospitals after the shooting.
Authorities say Cruz opened fire shortly before 3 p.m. after activating a fire alarm, sending people outside unaware of what was to come, officials said.
He fired shot after shot outside and then followed others running back inside the building, where people who'd heard the shots were taking shelter, officials said.
There, he roamed the halls, allegedly targeting those huddled in classrooms and then blending in with the students and staff evacuating the school. Investigators identified Cruz from school security videos and arrested him in the neighboring community of Coral Springs later in the day, the Broward County Sheriff's Office said.
Cruz had been expelled from the high school over disciplinary problems, Broward County Public Schools Superintendent Robert Runcie said, without providing specifics.
Former classmates said they were not surprised at the identity of the suspected shooter. Cruz loved showing off guns, student Eddie Bonilla told CNN affiliate WFOR.
"We actually, a lot of kids threw jokes around Iike that, saying that he's the one to shoot up the school, but it turns out everyone predicted it. It's crazy," Bonilla recalled.
An armed resource deputy was at the school's campus, but that deputy never encountered Cruz, Broward County Sheriff Scott Israel said.
'I saw my teacher dead on the floor'
At first, students were confused about the fire alarm because there had been a fire drill earlier that day. Investigators believe the suspect pulled the alarm to draw people out of classrooms and increase the number of casualties, a law enforcement source told CNN.
Some texted goodbyes to loved ones, fearful they wouldn't make it out alive. Others posted social media images of chairs overturned in classrooms and floors stained with blood.
Survivors described hearing anguished cries of the wounded. Kelsey Friend, a freshman, said she heard her teacher being shot dead in the doorway of their classroom while she and classmates hid near the teacher's desk.
Moments earlier, they had left the room for the fire alarm, but returned after hearing gunshots. The teacher unlocked the door, allowing students to get back in, she said.
"I had ran in, thinking (the teacher) was behind me, but he was not. ... I heard the gunshots, and I heard the shooter walk down the hallway.
"When we were all piled up by the desk ... my friend said, '(The teacher) is not moving. He is laying in the doorway.'"
Until that moment, she'd hoped it was an active-shooter drill, with police officers firing blanks, "until I saw my teacher dead on the floor."
In one cellphone video, a student cried, "Oh my God! Oh my God!" as gunshots popped in rapid succession in the background.
By the end of the rampage, 12 people were killed inside the building, two outside and one on the street. Two others shot at the school died at the hospital, Israel, the sheriff, said.
Relatives and others have been identifying some of the 17 students and adults who died.
The school's assistant football coach, Aaron Feis, and athletic director Chris Hixon were among the dead, according to the Stoneman Douglas football program and its spokeswoman, Denise Lehtio.
Feis threw himself in front of students as bullets flew Wednesday at his alma mater. It would become the final act of the coach and security guard, who suffered a gunshot wound and later died after he was rushed into surgery.
"He died the same way he lived -- he put himself second," Lehtio said. "He was a very kind soul, a very nice man. He died a hero."
Hixon gave his life "for our kids and probably helped prevent this from being a worse tragedy," Runcie, the superintendent, said.
Suspect was staying with acquaintance's family
The suspect used a .223 caliber, AR-15 style firearm in the shooting, which he bought in the past year after passing a background check, a source told CNN's Evan Perez.
Police are digging deeper into Cruz's digital footprint -- including a variety of gun and violence-related postings on social media sites -- and what they've found so far is "very, very disturbing," Israel said.
Postings under the name Nikolas Cruz included threatening comments under videos on YouTube and other sites, including "I whana shoot people with my AR-15" (sic), "I wanna die Fighting killing s**t ton of people" and "I am going to kill law enforcement one day they go after the good people."
Cruz was adopted, and his adoptive mother died in November. The family of someone he'd met at the high school allowed him to stay at their home, said Jim Lewis, the attorney for that family.
That family knew he had a gun, but "they had it locked up, and believed that that was going to be sufficient, that there wasn't going to be a problem," Lewis said.
Asked if the family had seen troubling signs, Lewis said they "saw some depression" over his adoptive mother's death.
"Obviously, he'd lost his mom. But they helped him get a job at a Dollar Tree store. They got him going to an adult education so he could try to get his GED and he seemed to be doing better," Lewis said.
CNN asked Lewis on Thursday whether the host family knew of any mental illness beyond depression.
"They didn't see that. They didn't see a mentally ill person, or they wouldn't have let him live under their (roof)," Lewis said.
"Nobody saw this kind of aggression or motive in this kid, that he would do anything like this," he said.
Kathie Blaine, a cousin of Cruz's adoptive mother, said he was adopted and she had never met him.
The adoptive mother -- her cousin Lynda Cruz -- died in November of pneumonia, while his adoptive father passed away years ago, according to Blaine.
Gun control debate revived
The school is closed for the rest of the week, Runcie, the superintendent, said. The district will offer grief counseling to students and their families.
The shooting has revived the debate over gun control.
One senator says congressional inaction is to blame for this latest massacre.
"This epidemic of mass slaughter, this scourge of school shooting after school shooting, it only happens here not because of coincidence, not because of bad luck, but as a consequence of our inaction," Sen. Chris Murphy, D-Connecticut, said in an address to the Senate on Wednesday. "We are responsible for a level of mass atrocity that happens in this country with zero parallel anywhere else."
President Donald Trump said Thursday he is making plans to visit Parkland to meet with families and local officials.
Trump, who also urged children across the country to seek help if they felt confused or scared, said he would meet with "the nation's governors and attorney generals where making our schools and our children safer will be our top priority."
"It is not enough to simply take actions that make us feel like we are making a difference; we must actually make that difference," Trump said.
Wednesday's shooting is at least the fourth at US middle and high schools this year. It comes more than five years after the deadliest such shooting -- at Connecticut's Sandy Hook Elementary School, where a gunman killed 20 children and six adults in December 2012.
Parkland, with a population of 31,000 in 2016, was named Florida's safest city last year. It had seven reported violent crimes and 186 property crimes the previous year, an analysis said.
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