The man accused of murdering 17 people in a mass shooting at a school in Florida is "sad," “mournful” and "remorseful," according to his lawyer.

Nikolas Cruz has now confessed carrying out one of the United States' deadliest school shootings, according to a sheriff's department report.

The 19-year-old has been remanded in custody after appearing in court.

He reportedly trained with a white nationalist militia group and the FBI has admitted it was warned about him in 2017.

"Broken"

His lawyer Melissa McNeill told reporters told reporters that he is a "broken human being" and is aware of what is going on.

He has been charged with murdering 17 students and teachers at Marjory Stoneman Douglas High School in Parkland, on the edge of the Everglades, on Wednesday afternoon.

Cruz was orphaned after his mother died last year and had previously been expelled from the school.

Police said after the shooting, he left the school and visited a Wal-Mart store before buying a drink from Subway then stopping at a McDonald's.

People hold up candles during a vigil for the victims of the attack, 15-02-2018. Image: AP Photo/Gerald Herbert

Attack

The 19-year-old was arrested 40 minutes later by police officer Michael Leonard after he saw someone matching the suspect's description and stopped him as he walked along a pavement nearby after the attack.

The report from the Broward County Sheriff's Office said the teenager told officers he "began shooting students that he saw in the hallways and on school grounds."

He "brought additional loaded magazines to the school campus and kept them hidden in a backpack until he got on campus to begin his assault," the report said.

Cruz set off the school's fire alarm to get all the students out and when they began to flee from his gunfire he discarded his AR-15 rifle and a vest so he could blend into the crowd, he told police.

Paramilitary militia

The teenager, who worked for discount retailer The Dollar Tree, is believed to have had paramilitary training in Tallahassee with white supremacist group Republic of Florida - with the group's leader saying he was a member.

However, a police spokesman in Tallahassee said there were "no known ties" between the suspect and the group.

Its leader, Jordan Jereb, told the Daily Beast: "I'm not trying to glorify it, but he was pretty efficient in what he did.

"He probably used that training to do what he did yesterday. Nobody I know told him to do that, he just freaked out."

He added that Cruz had "trouble with a girl" and believed the timing of the attack on Valentine's Day was not a coincidence.

Rich and Rachel Castleberry visit one of seventeen crosses after a candlelight vigil for the victims of the attack, 15-02-2018. Image: AP Photo/Gerald Herbert

Gun laws

In a news conference at the White House, US President Donald Trump said making schools safer will be his top priority - but he refused to address the issue of America's gun laws.

He sent his condolences to the families and friends of all of those killed and thanked those who responded to the shooting.

He raised the issue of mental health, urged students to go to teachers, parents or faith leaders to tell them when they think something is not right.

He said his top priority was to make schools safer.

Warning

The FBI has begun an extensive review of how it handled an apparent tip-off about Cruz, an official said.

YouTuber Ben Bennight said he told the FBI last year about a comment on the site under Cruz's name which read: "I'm going to be a professional school shooter."

Threats

Meanwhile a social media user who posted a picture of a man with a firearm and a mask with the caption “Florida Round 2” has been arrested in South Carolina.

Police said “variations of the post” have continued to be circulated online – with captions added warning people not to go to various schools throughout South Florida.

They said any violations of laws or threats to public safety will be prosecuted to the fullest extent of the law.

#SeeSomethingSaySomething #stonemanshooting pic.twitter.com/279xlDHD3Y

— Broward Sheriff (@browardsheriff) February 16, 2018

With reporting from Mick Staines