The leadership of the Next Generation Action Network drove all night from Baton Rouge, La., arriving in Dallas early on July 7, just hours before the start of their hastily arranged march that ended in the worst attack on law enforcement since 9/11.

Dominique Alexander, a 27-year-old Baptist preacher and the civil rights group’s founder, said that after the shooting deaths by police of Alton Sterling in Baton Rouge and Philando Castile in Falcon Heights, Minn., hundreds of messages poured into the group’s shared email and social media accounts, asking whether Dallas would hold a protest like those in Baton Rouge and Minneapolis. Black Lives Matter, which organized the other marches, doesn’t have a Dallas chapter.

“We should do this. We’ve got 24 hours. Let’s go,” Alexander recalled telling his companions on the trip, Next Generation’s attorney and its chief of staff.

So as they made their way to Baton Rouge to meet Sterling’s aunt, Alexander advertised on social media a rally the following day in downtown Dallas. Within hours, he said, more than 800 people had indicated they were coming, with another 800 marking themselves as “interested” on the march’s Facebook event page.

“You rally off that hype. If it’s just the right timing, you get a burst. You can get a stampede,” said Alexander, among the young activists who are leading a new civil rights movement in Dallas.

The Next Generation Action Network has held more than 50 protests against police brutality since the group was founded in August 2014 after the death of Michael Brown in Ferguson, Mo. Most of the protests have drawn small crowds.

So it surprised Alexander and the march’s co-organizer, fellow Baptist minister Jeff Hood, to see downtown Dallas flooded with people. They celebrated the proof of their movement’s vitality – until they heard shots fired.

At the end of the march, Micah Johnson, a 25-year-old Army veteran, trained his assault weapon on Dallas police, killing five officers and wounding nine others and two civilians. In an hours-long standoff with police that ended with his death, Johnson said that he wanted to kill officers, specifically white officers.

The shooting exposed a rift many people in Dallas thought had been healed decades ago.

Dallas, like many cities in America, remains divided along racial lines. Predominantly poor black and Hispanic neighborhoods are cut off from the rest of the city by three intersecting highways. A quarter of the city’s 1.3 million population lives in poverty, mostly on the city’s South Side, census data show.

South Dallas is home to some of the largest megachurches in the U.S., but large tracts remain empty and run-down, in stark contrast with the gleaming skyscrapers and expensive hotels north of Interstate 30.

Otherwise disparate groups of young local activists – many of whom grew up on the South Side and have lost family or friends in police-involved shootings – have coalesced around the issue of police brutality, reviving a civil rights movement in a city whose leaders often tout its progress with diversity.