The suspected Florida high gunman managed to unleash terror in just a few short minutes after wearing a school polo shirt to blend in and evading security as they were busy unlocking gates before he stormed in and shot dead 17 people.

Nikolas Cruz, 19, was wearing a maroon shirt with the logo from the Army Junior Reserve Officer Training Corps program at Marjory Stoneman Douglas High School when he was arrested on Wednesday following the massacre.

The teenager, who used to be a member of the ROTC marksmanship team before he was expelled, would have known they wore the polo to school on Wednesdays when he was hatching his deadly plan, according to some students.

It is believed he wore the shirt to blend in so he could sneak by undetected and then flee with the terrified students after he opened fire with an AR-15.

Nikolas Cruz, 19, was wearing a maroon shirt with the logo from the Army Junior Reserve Officer Training Corps program (above) at Marjory Stoneman Douglas High School when he was arrested on Wednesday following the massacre

Police have managed to put together a timeline of Cruz's deadly rampage based on the school's surveillance cameras and the multiple eyewitness accounts.

Students told the Miami Herald that Cruz, who arrived at the school at 2.06pm in an Uber, may have chosen that time given security would have been busy unlocking the various gates ahead of the dismissal bell.

Police have managed to put together a timeline of Cruz's deadly rampage based on the school's surveillance cameras and the multiple eyewitness accounts

Authorities are investigating initial reports that suggested Cruz triggered the school's fire alarm moments before he unleashed a barrage of gunfire so he could coax students into the hallways to maximize the number of casualties.

They haven't yet confirmed if Cruz pulled the alarm and the school's superintendent has since said it may have even been triggered by smoke from his assault rifle after he fired.

Details continue to emerge about the troubled teen gunman, including that he had mental health issues and had been reported to law enforcement before he used a legally purchased semiautomatic rifle in the 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. He reportedly left a suburban Palm Beach County mobile home where he had been staying after his mother's death because his benefactor gave him an ultimatum: you or the gun.

Students said Cruz may have chosen the afternoon to open fire because security would have been distraction unlocking the gates ahead of the dismissal bell. Students are pictured above evacuating during the 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

The Sun-Sentinel reported that 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.

'Mr Cruz has fresh cuts on both his arms,' the Florida DCF abuse hotline was told in August 2016.

'Mr Cruz stated he plans to go out and buy a gun. It is unknown what he is buying the gun for.'

DCF's investigation was completed on November 12. 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, two months after the DCF investigation concluded.

Records show he was suspended several times in the 2016-17 school year and was frequently absent. They also show Cruz attended at least six schools, including a school for students with emotional problems.

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

This photo posted on the Instagram account of Nikolas Cruz shows weapons lying on a bed. Cruz had spoken about his desire to buy a gun just days after he turned 18 years of age

Cruz had been diagnosed with autism, a neurological disorder that often leads to social awkwardness and isolation, and attention deficit hyperactivity disorder, or ADHD.

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.

President Donald Trump lashed out at the FBI Saturday night, saying the agency 'missed all of the many signals' in relation to Cruz - and argued they were 'spending too much time trying to prove Russian collusion with the Trump campaign.'

Trump said on Twitter: 'This is not acceptable. They are spending too much time trying to prove Russian collusion with the Trump campaign - there is no collusion. Get back to the basics and make us all proud!'

Pressure is now growing for tougher gun-control laws in the aftermath of the Florida shooting.