David Hogg, a survivor of the mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida, appeared in a House Judiciary Committee hearing on a possible assault weapons ban.
Nikolas Cruz took a rifle into Marjory Stoneman Douglas High School on Valentine's Day in 2018 and killed 17 students and staff members.
The shooter in Parkland used a Smith & Wesson M&P15 Sport II semi-automatic rifle. The M&P in the name stands for "Military & Police."
Hogg said that weapons like the one used in Parkland, the Bushmaster XM-15 semi-automatic rifle used in Buffalo, and the WASR-10 AK-47-style semi-automatic rifle used in El Paso didn't come to the United States from Mexico.
"They came from the US, and the shooters were inspired by racist, anti-black, anti-immigrant manifestos that rhyme with GOP talking points," he said on Twitter after the incident.
Cruz -- who was 19 at the time -- 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 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.
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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.
\u201cThe guns in Parkland, Buffalo, El Paso, didn't come from Mexico. They came from the US, and the shooters were inspired by racist, anti-black, anti-immigrant manifestos that rhyme with GOP talking points.\u201d— David Hogg \u262e\ufe0f (@David Hogg \u262e\ufe0f) 1658336302
With additional reporting by AFP