El Paso, Aug. 6 -- Authorities in two U.S. cities scoured leads in a pair of weekend mass shootings that killed 31, trying to piece together the motives that led two young men to unleash violence on innocent people in crowded public places.

In El Paso, Texas, the death toll creeped upward Monday from the shooting two days earlier at a Walmart store, with two additional victims succumbing to injuries. Police zeroed in on a racist screed posted online before the shooting to try to link it to the suspect. In Dayton, Ohio, even more questions remained about what spurred the suspected shooter to target a popular nightlife stretch in an attack that left the suspect's sister among the dead.

The back-to-back shootings hours apart and 1,300 mile...