PARKLAND, Fla. (AP) - The teenager accused of using a semi-automatic rifle to kill 17 people at a Florida high school confessed to carrying out one of the nation's deadliest school shootings and concealing extra ammunition in his backpack, according to a sheriff's department report.

The report from the Broward County Sheriff's Office says Nikolas Cruz told investigators that he shot students in the hallways and on the grounds of Marjory Stoneman Douglas High School, one of Florida's largest schools, with an enrollment of 3,000.

Cruz told officers he brought more loaded magazines to the school and kept them hidden in the backpack until he got on campus.

As students began to flee, he said, he decided to discard his AR-15 rifle and a vest he was wearing so he could blend in with the crowd. Police recovered the rifle and the vest.

MORE: Former student opens fire at Florida high school, killing 17

A day after the attack, a fuller portrait emerged of the suspect, a loner who had worked at a dollar store, trained with a white nationalist paramilitary group and posted photos of weapons on Instagram. At least one student said classmates joked that Cruz would "be the one to shoot up the school."

Cruz, a 19-year-old orphan whose mother died last year, was charged with murder Thursday in the attack at Marjory Stoneman Douglas High School in this sleepy community on the edge of the Everglades. It was the nation's deadliest school attack since a gunman assaulted an elementary school in Newtown, Connecticut, more than five years ago.

Meanwhile, students struggled to describe the violence that ripped through their classrooms on an ordinary day just before classes were to be dismissed.

Catarina Linden, a 16-year-old sophomore, said she was in an advanced math class Wednesday when the gunfire began.

"He shot the girl next to me," she said, adding that when she finally was able to leave the classroom, the air was foggy with gun smoke. "I stepped on so many shell casings. There were bodies on the ground, and there was blood everywhere."

Among the dead: a football coach who also worked as a security guard, a senior who planned to attend Lynn University, an athletic director who was active in his Roman Catholic church.

Some bodies remained inside the high school Thursday as authorities analyzed the crime scene. Thirteen wounded survivors were still hospitalized, including two in critical condition.

Authorities have not described any specific motive, except to say that Cruz had been kicked out of the high school, which has about 3,000 students and serves an affluent suburb where the median home price is nearly $600,000. Students who knew him described a volatile teenager whose strange behavior had caused others to end friendships with him.

Cruz was ordered held without bond at a brief court hearing. He wore an orange jumpsuit with his hands cuffed at his waist. His attorney did not contest the order and had her arm around Cruz during the short appearance.

Afterward, she called him a "broken human being" and added that she "had to have the exact same conversation that every parent in Broward County had to have with their children this morning."

Wednesday's shooting was the 17th incident of gunfire at an American school this year. Of the 17 incidents, one involved a suicide, two involved active shooters who killed students, two involved people killed in arguments and three involved people who were shot but survived. Nine involved no injuries at all.

As the criminal case began to take shape, President Donald Trump, in an address to the nation, promised to "tackle the difficult issue of mental health," but avoided any mention of guns. Trump, who owns a private club in Palm Beach, about 40 miles from Parkland, said he planned to visit the grieving community.

He did not answer shouted questions about guns as he left the room. At the Capitol, the usual divisions over gun laws were evident.

Trump, who did not speak publicly immediately after the shooting, weighed in on Twitter early Thursday, calling the suspect "mentally disturbed" and stressing that it was important to "report such instances to authorities, again and again!"

In the case of Cruz, at least one person did report him.

FBI agent Rob Lasky said the FBI investigated a 2017 YouTube comment that said "I'm going to be a professional school shooter." But the agency could not identify the person who made the comment, which was from an account using the name Nikolas Cruz. It was left on a YouTube video of a vlogger and bail bondsman from Louisiana named Ben Bennight.

In a Buzzfeed article, Bennight said he called the FBI, and agents came out to talk with him. They called him again Wednesday.

Officials were also investigating whether authorities missed other warning signs about Cruz' potentially violent nature.

He had been expelled from the school for "disciplinary reasons," said Broward County Sheriff Scott Israel, who said he did not know the specifics.

One student said Cruz had been abusive to his ex-girlfriend and that his expulsion was over a fight with her new boyfriend.

Math teacher Jim Gard told the Miami Herald that Cruz may have been identified as a potential threat before Wednesday's attack. Gard believes the school had sent out an email warning teachers that Cruz should not be allowed on campus with a backpack.

The leader of a white nationalist militia called the Republic of Florida said Cruz was a member of his group and participated in exercises in Tallahassee. Jordan Jereb told The Associated Press that he had only a brief interaction a few years ago with Cruz, who came across as "a normal Florida white guy."

The group wants Florida to become its own white ethno-state. Jereb said his organization holds "spontaneous random demonstrations" and tries not to participate in the modern world.

"We don't really endorse doing the things he did," Jereb said. "But at the same time, it's inevitable that people are going to go crazy because we live in an inherently sick society," he said, citing "hyper-egalitarianism" and feminism as some of society's ills.

He also said Cruz had "trouble with a girl" and that he believed the timing of the attack, on Valentine's Day, was not a coincidence.

When students streamed out of the school hours after the shooting, some of them clutched red and pink balloons that they had received from friends earlier in the day, before the shooting. Some kids said they initially thought the popping sounds that rang out at the end of the school day were from bursting balloons.

Student Jonathan Guimaraes, 17, told the Miami Herald that he had been in JROTC with Cruz.

"He was quiet, nice," Guimaraes said. "That's how he was able to blend in. He was wearing his JROTC uniform."

When he was arrested, Cruz had on a maroon polo shirt bearing an ROTC insignia and the school's eagle mascot.

Cruz was an orphan - his mother, Lynda Cruz died of pneumonia Nov. 1, according to neighbors, friends and family members who spoke to the Sun Sentinel.

Lynda Cruz and her husband, who died of a heart attack years ago, adopted Nikolas and his biological brother, Zachary, after the couple moved from Long Island in New York to Broward County.

After their mother died, the boys were left in the care of a family friend, family member Barbara Kumbatovich, of Long Island, said.

Unhappy there, Nikolas Cruz asked to move in with a friend's family in northwest Broward County. The family agreed, and Cruz moved in around Thanksgiving.

According to lawyer Jim Lewis, who represents but did not identify the family, they knew that Cruz owned the AR-15 but made him keep it locked up in a cabinet and never saw him go to a shooting range with it. He did have the key, however.

Two federal law enforcement officials said the Smith & Wesson M&P15 .223 was purchased legally at Sunrise Tactical Gear in Florida.

Cruz passed a background check and legally purchased the assault weapon from a licensed dealer in February 2017, a law enforcement official familiar with the investigation but not authorized to discuss it publicly told The Associated Press on condition of anonymity.