After mass shootings in Texas and Ohio killed at least 29 people within hours of each other, questions quickly arose about what exactly happened, who was responsible, and what might have prompted the attacks.
Here are the essential facts of Saturday's late-morning shooting in El Paso, Texas, and the early Sunday shooting in Dayton, Ohio:
What happened in each attack?
In El Paso, a major city on the border with Mexico, a lone gunman with an assault rifle opened fire on shoppers at a packed Walmart store, killing 20 people and wounding 26.
In Dayton, a lone shooter killed nine people and wounded 27 shortly after midnight in a nightlife district before almost immediately being shot dead by police, officials said. They said he was wearing body armor and had high-capacity magazines.
Who were the shooters?
Police said a man from Allen, Texas, was being held for the El Paso shooting. US media identified him as Patrick Crusius, a 21-year-old white man.
In Dayton, police declined to identify the shooter, saying their investigation was under way. A witness described the assailant as a white man, and CBS News quoted law enforcement sources as saying the shooter was 24-year-old Connor Betts of Bellbrook, Ohio.
What were their motivations?
The El Paso shooter had reportedly posted what police said was a manifesto shortly before the attack began. Its author railed against the "Hispanic invasion" of Texas. More than eight in 10 residents of the city, which was once part of Mexico, are of Hispanic descent.
There was so far no information on a possible motive in the Dayton shooting, though the shooter's weaponry suggested an intention to kill large numbers of people.
Who were the victims?
Mexican officials said three of the dead in El Paso came from Mexico, and six Mexicans were among the wounded. Mexicans frequently cross the border to shop in El Paso. News reports said the victims' ages ranged from two to 82 years.
There was little immediate information about the identities of the Dayton victims, but the fact the shooting occurred after midnight in a popular nightlife district suggests many or most would be young adults.