The suspected Florida high gunman allegedly repeatedly threatened to kill his ex-girlfriend's new boyfriend and physically attacked him twice in the months before he shot dead 17 people.

Nikolas Cruz, 19, is charged with carrying out the deadly massacre on Wednesday at Marjory Stoneman Douglas High School after he opened fire with an AR-15 rifle.

In the months leading up to the shooting, Cruz sent a series of threatening messages to 17-year-old Enea Sabadini after he started dating his ex-girlfriend, Buzzfeed reports.

'I'm going to f***ing kill you,' Cruz allegedly wrote to Sabadini. 'I am going to shoot you dead.

'I'm going to watch you bleed.'

Florida high school gunman Nikolas Cruz, 19, sent a series of threatening messages to 17-year-old Enea Sabadini (above) after he started dating his ex-girlfriend

In another message, Cruz wrote: 'You have no idea what I'm capable of' alongside a photo of a collection of weapons'

Cruz also repeatedly used homophobic and racial slurs in the message exchange, calling him a 'fa***t' and 'n****r'.

'You underground hispanic wall jumper, ill will f***ing destroy you,' he also wrote.

In another message, Cruz wrote: 'You have no idea what I'm capable of' alongside a photo of a collection of weapons.

Sabadini said he started dating Cruz's ex shortly after they broke up in August 2016.

Cruz is charged with carrying out the deadly massacre on Wednesday at Marjory Stoneman Douglas High School after he opened fire with an AR-15 rifle

Other messages accused Sabadini of stealing the girlfriend.

The ex-girlfriend has said she does not want to be identified.

Sabadini said he ended up breaking it off with Cruz's ex in December 2016 and the most recent threatening messages from Cruz were sent several months later in August 2017.

'He got upset about that and started sending messages to me on Instagram,' Sabadini said.

The teen said Cruz also picked two fights with him in September 2016 - one of which happened on a sidewalk outside the school, while the other happened during lunch in the courtyard.

He claims Cruz charged at him during the second fight, part of which was captured on video, and Sabadini said he ended up punching him several times in the face.

Both students were suspended over the incident.

Sabadini claims he and several other students reported Cruz's social media messages and other threatening behavior to school officials.

Cruz had already been expelled from the school when the messages were sent, but Sabadini said he doesn't know what led to his dismissal.

In another message to Sabadini, Cruz wrote: 'You have no idea what I'm capable of' alongside this photo of a collection of weapons. The photo was also posted on Cruz's Instagram page

From a mosaic of public records, interviews with friends and family and online interactions, it appears Cruz was unstable and violent to himself and those around him

Cruz (pictured in court on Thursday) is charged with 17 counts of premeditated murder after killing 14 students and three staff members

Harrowing details continue to emerge about Cruz, including that he had mental health issues and had been reported to law enforcement before he used a legally purchased semiautomatic rifle in last week's massacre.

From a mosaic of public records, interviews with friends and family and online interactions, it appears Cruz was unstable and violent to himself and those around him - and that when notified about his threatening behavior, law enforcement did little to stop it.

Cruz's mother died in November and his father died years ago.

Florida's Department of Children and Families investigated when Cruz posted a video on the social media network Snapchat showing him cutting his arms in 2016.

The agency concluded Cruz had not been mistreated by his mother, was receiving adequate care from a mental health counselor and was attending school.

At school, Cruz routinely fought with teachers, was accused of swearing at staff and was referred for a 'threat assessment' in January 2017.

Records show he was suspended several times in the 2016-17 school year and was frequently absent.

The FBI said a person close to Cruz called the FBI's tip line and provided information about Cruz's weapons and his erratic behavior.

The caller was concerned Cruz could attack a school. The agency acknowledged the tip should have been shared with the FBI's Miami office and investigated, but it was not.