A young man who gunned down 17 people at his former high school in Florida went on trial in the southern US state on Monday, with jurors set to hand down either the death penalty or a life sentence.

Nikolas Cruz took an AR-15 assault rifle into Marjory Stoneman Douglas High School on Valentine's Day in 2018 and killed 17 students and staff members.

Cruz, who was 19 at the time of the shooting, has already pleaded guilty to 17 counts of first-degree murder and 17 counts of attempted murder for those wounded during the massacre.

The trial in Fort Lauderdale -- a rare instance of a mass shooter facing a jury, as they often either take their own lives or are killed by police -- is being held to determine his sentence.

The death penalty requires a unanimous decision by the jury. Cruz will otherwise be handed life without parole.

The Florida shooting stunned a country accustomed to gun violence and sparked new efforts, led by students from the school itself, to get lawmakers to pass tougher gun control laws.

Parkland survivors founded "March for Our Lives," organizing a rally that drew hundreds of thousands of people to Washington, DC in 2018.

Thousands turned out for demonstrations organized by the group last month following two other mass shootings: one at a Texas elementary school that killed 19 young children and two teachers, and another at a New York supermarket that left 10 Black people dead.

Those shootings helped galvanize support for the first significant federal bill on gun safety in decades.

President Joe Biden signed the bill into law in late June. It included enhanced background checks for younger buyers and federal cash for states introducing "red flag" laws that allow courts to temporarily remove weapons from people who are considered a threat.

Nine-minute massacre 

But the measure fell far short of steps Biden had called for, including an assault weapons ban.

Cruz bought the weapon he used in the 2018 attack legally, despite having a documented history of mental health problems.

Expelled from school for disciplinary reasons, Cruz was known to be fixated on firearms -- and had reportedly been identified as a potential threat to his classmates.

On the day of the attack, Cruz arrived at the school in an Uber, began shooting indiscriminately at students and staff, and fled nine minutes later, leaving behind a scene of carnage.

He was arrested nearby shortly afterward.

Footage recovered from his phone showed he had filmed his plans to attack his former school, saying his goal was to kill "at least 20 people."

Cruz told a detective after his arrest that he heard demons ordering him to "buy weapons, kill animals and destroy everything."

The US Justice Department in March announced that it had reached a $127.5 million settlement with survivors and relatives of victims of the shooting, settling all 40 related civil cases.

In lawsuits, survivors and relatives accused the Federal Bureau of Investigation of negligence for failing to act on tips received prior to the attack that Cruz was dangerous.

According to the Gun Violence Archive website, more than 24,000 people have been killed by firearms in the United States so far this year, including more than 13,000 who died by suicide.