Mr. Obama made the comment at a press conference in Warsaw, Poland, where he also issued another call for gun control in the wake of the Dallas shootings.

“I think it’s very hard to untangle the motives of this shooter,” Mr. Obama said of the gunman, Micah X. Johnson. “By definition if you shoot people who pose no threat to you, you have a troubled mind.”

Dallas police said Johnson told them before he was killed that he specifically was targeting white police officers for killing, and he was upset about the Black Lives Matter movement.