The 19-year-old accused of gunning down 17 people at his former high school admitted to carrying out the shooting rampage, authorities said in court papers filed today.

Police wrote that Nikolas Cruz, who has been charged with 17 counts of premeditated murder, told officers that he walked into Marjory Stoneman Douglas High School on Wednesday wielding an AR-15 and began shooting students in the hallways and outside on the school's grounds.

Once students began to flee the carnage, Cruz dropped his rifle and vest packed with additional ammunition "so he could blend into the crowd," an officer wrote in a probable cause affidavit. Cruz had taken an Uber to the school, officials wrote, so he fled on foot along with those running from the gunfire.

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Broward County Sheriff Scott Israel said Cruz stopped at fast food restaurants after the attack. The sheriff says he headed to a Wal-Mart and bought a drink at a Subway restaurant before walking to a McDonald's.

He ws confronted by a police officer on a residential street and taken into custody about 40 minutes after leaving the McDonald's.

The revelations came as police vowed to make sure that "justice is served" after one of the country's deadliest school shootings. A day after the Valentine's Day shooting rampage, authorities had not publicly announced a motivation for the carnage, but they were digging into elements of Cruz's troubled past, including a pattern of disciplinary issues and unnerving behaviour.

Nicolas Cruz has been charged with the deaths of 17 high school students in Floria. (Photo \ Getty Images)

Also on Thursday, FBI investigators were pursuing information suggesting that Nikolas Cruz, the 19-year-old suspect charged with the shooting, might have been associated with a Florida-based white supremacist group. But agents were still trying to determine the extent of his involvement with the group, if any, according to a law enforcement official who asked not to be identified discussing an ongoing investigation.

It was not immediately clear what connection there may be between the shooting rampage and Cruz's possible connections to the group.

The Anti-Defamation League reported Thursday that a spokesperson for the white supremacist group said Cruz participated in some of their training exercises. The spokesperson told the ADL that the group did not order or want Cruz to carry out any attack like the school rampage. The Washington Post was not immediately able to reach several of the group's members on Thursday.

What unfolded inside the high school was a grimly familiar sight, as another school became transformed into a war zone in an otherwise quiet suburban community transformed by a hail of bullets and a torrent of anguish.

In the aftermath, investigators have sifted through the background of Cruz, who had been expelled from the same school now turned into a crime scene. He had a history of disciplinary issues and a fascination with guns, people who knew him said.

A witness whose name is redacted from the report filed in court Thursday said they recognised Cruz as a troubled former student when he arrived at Douglas before the shooting, carrying a black duffel bag and wearing a black backpack. Cruz told police he had extra loaded magazines in the backpack, they wrote in the report.

The witness quoted in the report said they radioed a coworker to tell them Cruz was approaching, and within a minute, they heard gunshots and called a "Code Red," which announced an emergency.

Cruz's attorneys did not specifically say Thursday that he had admitted to the shooting, nor did they explicitly deny his involvement.

Early Thursday, Cruz was booked on 17 counts of "murder premeditated." He briefly appeared in court on Thursday and was ordered held without bond. Broward County Sheriff Scott Israel vowed that law enforcement officials would make sure Cruz was convicted on all charges.

"This community is hurting right now. . . Today's a day of healing," Israel said at a briefing Thursday morning. "Today's a day of mourning."

Israel said that police have identified all of the victims and planned to release their names later in the day. Some were students, but at least one staff member - a beloved football coach who had attended the school before returning to work there - was among the dead.

In addition to the 17 killed in the school and outside, another 15 were wounded, authorities said. Three of those remained in critical condition Thursday, while some others were still hospitalised.

Israel said that it was "a pretty good assumption" that the shooter had unspecified mental-health issues.

During a news conference after Cruz's court hearing, the public defender representing him described the 19-year-old as a "broken young man " who is " very saddened" by what happened. Cruz is on suicide watch, he added.

"This is an emotionally broken young man," Gordon Weeks, the public defender, told reporters. "He has been through a lot of trauma. He has suffered significant mental illness, and significant mental trauma."

But while school officials, students and others who knew him had sensed that something was off with Cruz before the attack, none of that was enough to stop the teenager from purchasing the gun officials said was used in the attack.

Cruz bought the AR-15 himself legally, and so far it is the only gun authorities have recovered as part of the investigation, said Peter J. Forcelli, special agent in charge of the Miami field division of the Bureau of Alcohol, Tobacco, Firearms and Explosives. The court filing Thursday said Cruz bought the gun in February 2017.

In his social media postings, Cruz has been seen wielding other firearms, so officials continue looking for any additional weapons, but they have not found any so far, Forcelli said in an interview Thursday morning. Investigators are also reaching out to gun shops across the region to see if Cruz had attempted to buy other weapons.

Federal authorities were looking Thursday into whether Cruz had come up on their radar before. The FBI said that agents investigated a comment on YouTube last year that threatened a school shooting but were unable to identify the person who posted it.