The teenager accused of a shooting massacre at a US high school is an orphan who regularly posted images of guns and graphic violence to social media and was reportedly identified as a security threat by the school months before the attack.

Nikolas de Jesus Cruz, 19, is accused of opening fire on students and teachers at the Marjory Stoneman Douglas High School in Parkland, Florida, on Wednesday (US time), killing 17 people and leaving up to 50 injured.

Cruz, who called himself “Annihilator” on one of his now-deleted Instagram accounts, had been expelled from the school for disciplinary reasons, Broward County schools Superintendent Robert Runcie revealed.

When news broke that Marjory Stoneman was under siege, “honestly, all people were saying was that it was going to be him [Cruz]”, said one student.

“Everyone predicted it,” the student told local network News4JAX.

Former classmate Joshua Charo, 16, told the Miami Herald: “All he would talk about is guns, knives and hunting.

“I can’t say I was shocked.”

"Everyone predicted it," a student said about the #Parkland shooting. pic.twitter.com/mbzBIw8iUa

— Vic Micolucci WJXT (@WJXTvic) February 14, 2018

Giovonni Watford, 17, was enrolled with Cruz in a Junior Reserve Officer Training Corps program, which prepares students for the military.

“He was off,” Mr Watford told BuzzFeed News. “He was super stressed out all the time and talked about guns a lot.”

One student told WSVN that Cruz made him nervous because he was obsessed with guns and had shown pictures of them on his phone.

“He’s been a troubled kid and he’s always had a certain amount of issues going on,” the unnamed student said.

“He shot guns because he felt it gave him, I guess, an exhilarating feeling.”

A family member who spoke to ABC News said Cruz was adopted as an infant by Roger and Lynda Cruz. Roger passed away in 2005 of a heart attack and Lynda died last year of the flu.

A photo from one of Cruz’s now-deleted Instagram accounts.

“I know he had his own problems, but I never thought he would be capable of this,” Fernando Santos, a friend of Cruz’s who interacted with him over Instagram, said.

“I did know he really did like guns a lot, but I didn’t think he would cause such tragedy.”

Helen Pasciolla, a neighbour from the Cruz family’s former Parkland neighborhood, told the New York Times that Cruz and his brother, Zachary, who are both adopted, had behavioural problems.

Broward County Sheriff Scott Israel described Cruz’s social media activity in the lead-up to the massacre as “disturbing”.

Cruz had at least two Instagram pages that showed a fixation with knives and guns. In 2016, he posted a photo of a shotgun advertisement, writing: “I plan on getting this.”

At school, Cruz reportedly complained often about bullying. He said “how tired he was of everyone picking on him and the staff doing nothing about it”, 2016 graduate Mike Watford told BuzzFeed News.

“He had a lot of problems in middle school,” fellow student Nicholas Coke told WSVN, describing Cruz as a “loner” who once kicked out a glass window.

Because of his alleged issues, Cruz was banned last year from carrying a backpack on campus, according to Jim Gard, a former maths teacher at the school.

“There were problems with him last year threatening students, and I guess he was asked to leave campus,” he told the Miami Herald.

Mr Gard’s claims that school officials emailed teachers last year warning them about Cruz were denied by Mr Runcie.

“Potentially there could have been signs out there,” he told a press conference. “But we didn’t have any warning or phone calls or threats that were made.”

But Mr Gard, who taught Cruz in 2016, described him as “a quiet kid in class. I never had any problems with Nick”.