This is a developing story and may be updated.

Within 13 hours in two U.S. cities—El Paso, Texas and Dayton, Ohio—at least 29 people were killed in mass shootings on Saturday and Sunday.

Police in Dayton, Ohio were gathering information on Sunday morning about a shooting that took place in the city's downtown Oregon neighborhood at about 1:00am. Nine people were killed by a gunman who used what one eyewitness described as an assault rifle and wore body armor, and at least 27 were injured.

In Washington, D.C. just a few hours before the shooting, hundreds of protesters representing the national gun control group Moms Demand Action marched from the White House to the Capitol to demand legislative action following Saturday morning's shooting at a Walmart in El Paso, Texas.

Hundreds of @momsdemand and #StudentsDemandAction volunteers and survivors of gun violence spontaneously making their voices heard in DC. #NotOneMore #GSU19 pic.twitter.com/08OQP3QyEe — nick suplina (@nicksuplina) August 4, 2019

A number of observers on social media expressed grief over the two shootings taking place within a day of one another.

We live in a country where the mass shooting that took place yesterday in El Paso, Texas is not the top story because there was another mass shooting overnight in Dayton, Ohio. #MakeAmericaSaneAgain pic.twitter.com/0oPnmOLYtf — G.E. Anderson (@g_e_anderson) August 4, 2019

How many others did what I did? -- fell asleep with the latest news out of El Paso on the TV, slept fitfully for about 6 hours and woke up at 5 a.m. startled to see carnage in Dayton on the screen. In America, the nightmares come when you wake up — Will Bunch (@Will_Bunch) August 4, 2019

Still grieving over the mass shooting in El Paso. My heart breaks again for the victims of another mass shooting in Dayton. The policies of @realDonaldTrump & his enablers have not made us safer. Time to give others a chance. Vote out those who do nothing about gun violence. https://t.co/fax4FyhOaL — Ted Lieu (@tedlieu) August 4, 2019

Investigators on Sunday were examining an anti-immigration "manifesto" posted online by a 21-year-old gunman who opened fire in El Paso late Saturday morning, killing at least 20 people and injuring 26.

"If reports are true, this senseless attack cannot be separated from the escalating politics of hate and fear," said Ernest Coverson, head of Amnesty International USA's gun control campaign. "This is simply unacceptable. Everyone has the human right to live in safety, whether they are going to the mall or walking down the street in their neighborhood."

The shooter fired multiple rounds with an AK-47-style rifle, walking through the store and shooting shoppers aisle-by-aisle.

The manifesto was posted on the online forum 8chan less than 20 minutes before the first 911 call was made about the shooting, according to the New York Times. The forum was the same one on which the gunman in the March mass shooting in Christchurch, New Zealand posted his own statement about his intent to kill Muslims and immigrants, shortly before he killed 51 people at two mosques.

The El Paso shooter expressed support for the Christchurch gunman and reportedly said his own goal was to stop the "Hispanic invasion of Texas" with the shooting in the largely Latino border city.

The man lived outside Dallas and made the nine-hour trip because he believed "the heavy Hispanic population in Texas will make us a Democrat stronghold," according to the statement. He expressed support for the white supremacist theory of "the great replacement" which argues that so-called "elites" are attempting to replace white Europeans with immigrants from the Middle East and Africa.

"This is about hate," Rep. Veronica Escobar (D-Texas), who represents El Paso, said in a statement. "The manifesto narrative is fueled by hate, and it is fueled by racism and bigotry and division."

On social media, just as many have vehemently rejected the "thoughts and prayers" of politicians who accept thousands of dollars in campaign donations from the NRA and refuse to fight for broadly-popular gun control reforms, some critics directed ire at another perennial post-shooting sentiment, expressed by Texas Gov. Greg Abbott hours after the El Paso shooting—that better mental healthcare is the answer to the United States' epidemic of gun violence.

The causes of the shooting in El Paso, many said, were the shooter's easy access to guns and the growth of the white supremacist viewpoints he allegedly subscribed to—as well as support for such views by President Donald Trump and other elected officials.

BULLSHIT: The El Paso gunman drove NINE HOURS to the Texas border to kill brown people. White supremacy is not a mental illness. It’s an ideology encouraged by the President and too many of our leaders, and it’s made deadly in the US via easy access to arsenals and ammunition. https://t.co/yyBb5dYbNg — Shannon Watts (@shannonrwatts) August 4, 2019