Parkland, February 16: 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 carried extra ammunition in his backpack, according to a sheriff's department report released today.

Nikolas Cruz told investigators that he shot students in the hallways and on the grounds of Marjory Stoneman Douglas High School in Parkland, north of Miami, the report from the Broward County Sheriff's Office said.

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

The gunman fired into five classrooms -- four on the first floor of the school and one on the second floor, Sheriff Scott Israel said.

The shooting lasted for three minutes. When he was done firing, the assailant went to the third floor and dropped his AR-15 rifle and the backpack containing the ammunition. He then ran out of the building and attempted to blend in with fleeing students, Israel said.

After the rampage, the suspect headed to a Wal-Mart and bought a drink at a Subway restaurant before walking to a McDonald's. He was taken into custody about 40 minutes after leaving the McDonald's, the sheriff said.

A day after the attack, a fuller portrait emerged of the shooter, a loner who had worked at a dollar store, joined the school's ROTC program 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."

The 19-year-old orphan whose mother died last year was charged with murder today in the assault that devastated this sleepy community on the edge of the Everglades. It was the nation's deadliest school attack since a gunman targeted an elementary school in Newtown, Connecticut, more than five years ago.

Meanwhile, students struggled to describe the violence that ripped through their classrooms just before the school day ended.

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

The last of the bodies were removed from the high school today after authorities analysed the crime scene. Thirteen wounded survivors were still hospitalised, including two in critical condition.

Authorities have not offered 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 USD 600,000. Students who knew him described a volatile teenager whose strange behaviour 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 had her arm around Cruz during the short appearance. Afterward, she called him a "broken human being."