Nicholas Cruz, a 17-year-old former schoolboy who was arrested an hour after the incident as a suspect in the shootings, confessed to the massacre, which killed 19 people at a Florida school on Wednesday night.
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The 19-year-old said he arrived at the campus and started shooting students before he dropped his gun and fled, according to a court document.
"Cruz said he was the perpetrator who entered the school armed with an AR-15 and started shooting at students he saw in the hallways and school grounds."
He said he was carrying extra ammunition in a black bag and backpack.
After committing the crime, he entered a Walmart store and then a McDonalds store, and was eventually located by police and arrested an hour after the attack.
The attack on Marjory Stoneman Douglas High School in Parkland, Florida, is the deadliest incident at a US school since 2012.