In one of the deadliest racist massacres in recent American history, a white 18-year-old has been accused of shooting and killing 10 people at a Buffalo supermarket on Saturday. Authorities said Payton Gendron of Conklin, New York, shot 11 Black people and two white people in a rampage that he broadcast live.

A 180-page manifesto believed to have been posted on the internet by Gendron before the attacks focused on “replacement theory,” a white-supremacist belief that non-whites will eventually replace white people because they have higher birth rates, according to a copy viewed by ABC News.

“This individual came here with the express purpose of taking as many Black lives as he could,” Buffalo Mayor Byron Brown said at a news conference Sunday.

Since taking office in August, New York Governor Kathy Hochul has faced several natural and man-made disasters, ranging from deadly Hurricane Ida to the recent subway shootings in Brooklyn. But for the Buffalo native, the racial-motivated mass shooting in her hometown is personal.

In an interview on ABC News on Sunday morning, Hochul expressed her grief and outrage: “Our hearts re broken—and I am angry. An attack on one of us is an attack on all of us. I will leave no stone unturned to protect the people of this community.”

Democrats lashed out against Republicans who are traditionally strong advocates of the Second Amendment, including the GOP’s third-highest ranking member in the House, upstate New York Rep. Elise Stefanik.

“Did you know: @EliseStefanik pushes white replacement theory?” Rep. Adam Kinzinger (R-Ill.) tweeted on Saturday, referring to criticism of her congressional campaign’s Facebook ads hyping fears of a “permanent election insurrection.”

Stefanik, known as a moderate Republican turned Trump acolyte, tweeted a message of condolence upon hearing the news but has not commented on Kinzinger’s allegation.

“We pray for their families. But after we pray—after we get up off of our knees—we've got to demand change. We've got to demand justice,” New York State Attorney General Letitia James said while attending church services in Buffalo on Sunday morning. “This was domestic terrorism, plain and simple.”

For Hochul, the massacre reflected a failure not just to limit access to guns but to curb the ability to openly share and distribute hate speech.

The governor told ABC that the heads of technology companies “need to be held accountable and assure all of us that they’re taking every step humanly possible to be able to monitor this information.”

“How these depraved ideas are fermenting on social media–it’s spreading like a virus now,” she said, adding that a lack of oversight could lead to others emulating the shooter.

The Buffalo shooting prompted the New York Police Department to provide increased security at Black churches around New York City “in the event of any copycat,” the NYPD said in a statement.

“While we assess there is no threat to New York City stemming from this incident,” the NYPD said in its statement, “out of an abundance of caution, we have shifted counterterrorism and patrol resources to give special attention to a number of locations and areas including major houses of worship in communities of color.”