In the days before the Dallas massacre, Aesha Rasheed, 39, an activist in New Orleans, felt that at long last, white and black America were watching the same images with the same horror: two Louisiana police officers tackling and then shooting Alton Sterling, 37, at point-blank range; the slumped, blood-soaked body of Philando Castile, 32, after a Minnesota police officer shot him through a car window, with his girlfriend and her daughter sitting inches away.

“It seemed like a national consciousness was sinking in,” Ms. Rasheed said.

After the massacre in Dallas, she said, “it turned on a dime.”

She now worries that the episodes involving black men may be overshadowed and overlooked.

“Does this get ignored?” she asked. “Do five officers take center stage?”