A Sunday night shooting at an outdoor country music festival in Las Vegas has become the deadliest in United States history, leaving 50 people dead and hundreds more injured. The mother of a victim of the Sandy Hook Elementary School shooting — which left 20 children and six adults dead in 2012 — tweeted Monday that Sunday’s event is on “every congressperson who said in ‘13: There is simply nothing we could do.”

Nelba Márquez-Greene, whose 6-year-old daughter was killed during the shooting in Newtown, Connecticut, wrote on Twitter Monday that she was “heartbroken” by news of the most recent rampage Route 91 Harvest festival at the Las Vegas Village.

“Every day, I am stunned by the level of trauma (direct or vicarious) congress is willing to make us suffer through. Their lack of courage and/or ability to take meaningful action on issues that most matter: healthcare, violence, climate, etc is outrageous,” she said. “As a mom who had to bury a child- I could care less about perp color. But how come we never talk about angry White men w/guns?”

“I will also say this: I lost Ana in Newtown which means even though we aren’t white I had the sympathy of the world,” she added. “Guess what folks? Gun violence and grief hurt in EVERY zip code. In every color. Grieving mothers need your help.”

Márquez-Greene went on to write that “[C]ongress people” had “implied that we hadn’t prayed hard enough,” adding that there’s “no such thing as being in the wrong place at the wrong time when a 6 y.o. gets shot.”

Stephen Paddock, 64, of Mesquite, Nevada, is the suspected shooter in Sunday's incident. According to reports, the gunman opened fire on Sunday’s festival from the 32nd floor of the nearby Mandalay Bay Resort and Casino, according to reports. Fifty people were killed and an estimated 406 people were transported to hospitals following the shooting, according to Clark County officials.

The Associated Press reported Monday that the shooter killed himself prior to police entering his hotel room, in which authorities reportedly found at least eight guns.

Las Vegas People scramble for shelter at the Route 91 Harvest country music festival after a gunman opened fire in Las Vegas on Sunday, October 1, leaving 59 people dead and more than 400 injured. Photo: Photo by David Becker/Getty Images