The 19-year-old orphan accused of perpetrating America’s deadliest school shooting in five years has appeared in a Florida court charged with 17 counts of premeditated murder.

Wearing orange overalls and hands cuffed to his waist, Nikolas Cruz appeared at a brief hearing in the Broward County Court on Thursday local time, where Judge Kim Mollica ordered he be held without bond.

Outside court, defence lawyer Melisa McNeill said Mr Cruz was sad, mournful and remorseful, the ABC reported.

“He is fully aware of what is going on. He’s just a broken human being,” Ms McNeill said.

Police say Cruz, equipped with smoke grenades and multiple magazines of ammunition, allegedly opened fire with a semi-automatic weapon on Wednesday (local time) at the Marjory Stoneman Douglas High School in Florida, killing 17 people and sending hundreds of students fleeing into the streets.

A day after the attack, a fuller portrait emerged of the suspect, a loner who had worked at a dollar store, reportedly 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”.

Authorities have not described any specific motive, except to say that Cruz had been kicked out of the high school, which has about 3000 students and serves an affluent suburb where the median home price is nearly $US600,000 ($756,000). Students who knew him described a volatile teenager whose strange behaviour had caused others to end friendships with him.

It was the nation’s deadliest school shooting since 26 people died when a gunman attacked an elementary school in Newtown, Connecticut, in December 2012.

Cruz was taken into custody without a fight about an hour after the shooting in a neighbourhood about 2km away.

Once in custody, Cruz reportedly confessed to carrying out the 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 Cruz allegedly told investigators that he shot students in the hallways and on the grounds of the high school.

He allegedly 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.

Some bodies remained inside the high school on Thursday as authorities analysed the crime scene.

Thirteen wounded survivors remain in hospital, including two in a critical condition.

— with agencies