Steps from where the shooting began, Dominique Alexander and Dominique Torres, leaders of the Next Generation Action Network, tried to make sense of how the Black Lives Matter protest they had planned had turned into a tragedy.

Twelve police officers had been shot, five fatally, at a protest they called for on their group’s Facebook page. Strangers on social media were telling them it was their fault. The lieutenant governor of Texas had referred to protesters as hypocrites for looking to police for protection while protesting police brutality.

They were carrying an emotional weight that none had fully expected. And now they had to figure out how to keep going.

Alexander, 27, said he needed to mourn the deaths of the police officers, but he didn’t want to obscure the larger issue of racial disparities in law enforcement.

“This is not going to stop our movement,” Alexander told reporters. “In fact, we want it to strengthen it. No one deserves to get shot unjustly — not police officers, not black men. A shooting in Dallas doesn’t mean the problem doesn’t exist anymore.”

People in Dallas, St. Paul, Minn., and across the nation grapple with how to move on from the deaths of two black men at the hands of police as well as the loss of five police officers all in one week's time. (Whitney Shefte,McKenna Ewen,Dalton Bennett,Ashleigh Joplin/The Washington Post)

Black Lives Matter is a diffuse movement that started with a hashtag on social media three years ago this week. It has increased awareness of police killings of black people and led to reforms in how police behavior is monitored, including through data collection and body cameras. Now it faces its greatest challenge.

Activists associated with Black Lives Matter refused to retreat from their slogan this weekend, staging large protests in cities around the country, including Atlanta, Phoenix and San Francisco. Leaders decried the loss of life in Dallas and strongly rejected any ties between the gunman and their activism.

“He wasn’t a protester,” said Johnetta Elzie, a St. Louis native who emerged as a leader of the movement during protests following the death of Michael Brown, an unarmed black man in Ferguson, Mo., in August 2014. “People are conflating the message with being anti-police and its anti-police violence.”

Black Lives Matter was born as a cry of protest on July 13, 2013, after George Zimmerman was acquitted in the shooting death of Trayvon Martin, an unarmed black youth walking home in Florida. That night, activist Alicia Garza wrote: “We don’t deserve to be killed with impunity. We need to love ourselves and fight for a world where black lives matter.” Her friend Patrisse Cullors attached a hashtag to the words, and with a third woman they later founded a #BlackLivesMatter organization, although they don’t try to control who uses the phrase.

The movement has had a huge impact despite not having a central leader. What started as a social-media phenomenon gained much more attention during the months of protests following Brown’s killing. Videos of police shootings now routinely circulate under the #BlackLivesMatter tag and go viral.

Protests took place in cities across the country on Friday to condemn police brutality. In Atlanta, some demonstrators blocked a highway and in Phoenix police used tear gas and pepper spray to break up a protest. People in Dallas gathered outside of the police headquarters to remember the five officers killed in an ambush. (Claritza Jimenez/The Washington Post)

None has evoked more emotion than last week’s live-streaming of the aftermath of a police officer shooting Philando Castile in Minnesota. Following his death, the outcry, which included a statement by President Obama, made clear that activists were succeeding in their mission of convincing the world that black people were being targeted by police because of their race.

Civil rights lawyer Judith A. Browne Dianis described last week as “the week from hell” for Black Lives Matter. First came “the deaths of Alton Sterling and Philando Castile as the constant reminder of the work that we have to do,” said Dianis, director of the Advancement Project, a liberal nonprofit organization that coordinates with groups in the movement. The shootings in Dallas followed and “make it harder for us to continue on in doing the work of police accountability because it makes the narrative more complicated. . . . And in some ways it felt like the pain that we have felt [for the deaths of Sterling and Castile] was swept under the rug.”

She sounded weary, as did a half dozen other Black Lives Matter leaders. In interviews, nearly all described themselves as tired.

“What is happening is just so traumatizing, and my heart is just so heavy,” said Clifton Kinnie, who organized high school students during protests near his home town of St. Louis. “Being in the movement has caused me to get to a point where my heart is so heavy. Trayvon Martin. Michael Brown. Tamir Rice. Sandra Bland. . . . The past 72 hours my phone was on do not disturb. My spirit was not broken, but like many in the community I am tired.”

As the Black Lives Matter anniversary approaches, activists involved have been reassessing their strategies. They will have to do so while contending with sharpened criticism of their movement. Texas Lt. Governor Dan Patrick said during an interview on Fox News that he blames “people on social media with their hatred towards police” for the shooting, and called protesters in Dallas “hypocrites” for running when the sniper opened fire and “expecting men and women in blue to turn around and protect them.” The Drudge Report posted a headline saying, “Black Lives Kills Four Police Officers.”

In a statement late last week, the official Black Lives Matter organization that Garza and Cullors founded, which has 40 chapters and represents a portion of the movement, predicted that “there are some who would use these events to stifle a movement for change and quicken the demise of a vibrant discourse on the human rights of Black Americans.”

The leaders of Black Lives Matter fear the negative commentary will weaken the movement’s influence on the national debate after a period in which activists had begun to feel more powerful. Since 2014, more than two dozen states have enacted laws addressing police violence, including the collection of data on police shootings, and more bills dealing with the issue are being considered. State leaders quickly remanded investigations of Sterling’s and Castile’s deaths to federal authorities, unlike Brown’s case.

“There is an entirely new generation of black people on the front lines for justice and dignity and respect for black people. We have opened conversation in the political sphere, the art world, Hollywood,” Cullors said. “People have been really pushed to grapple with what it means to show up for black lives.”

The Dallas protest began when Alexander and others in the Next Generation Action Network were driving back to Dallas from a protest in Baton Rouge, where they had visited the family of Alton Sterling. Then the video showing Castile, taken by his girlfriend, Diamond Reynolds, started popping up on their phones. It added extra layers of urgency, outrage and mourning.

They drafted a news release and a Facebook announcement. “We don’t do a lot of planning,” Alexander said. “You just put something out there and wait for people to come.”

More than 1,200 people checked into the protest on Facebook. And everything was peaceful — police officers posed for pictures with protesters, and some even shared hugs — until it wasn’t.

Alexander emphasized that their relationship with these police officers was not adversarial. Police contact them whenever they have planned a protest, and the same officers show up to clear the streets and protect them from violence.

“This is not about protesters versus the police,” added Simone Bridges, 25. “We want the police to be safe, and we want to be treated with dignity. We just want them to do their jobs.”

Alexander and Torres had been so busy talking all day that they did not even see the photos of the officers who were shot. A reporter showed him photos of the officers, each a new revelation. Patrick Zamarripa: “I seen him.” Lorne Ahrens: “He was the one leading in the front. I know him. Sad.” Michael Krol: “I’ve seen him, too.” Michael Smith: “He’s out there all the time.” Brent Thompson: “Yeah. Him.”

Alexander went quiet, and his eyes started to glisten.

“I knew all five,” he said, quietly. “All five.”

“We’re going to hope to be at the table to help the police get better,” Torres added. “But we know they need their space now, and they need time to mourn.”

So did they.