Illegal crossings are down sharply on the southwest border, but the number of people dying is climbing. At least 412 migrants were found dead in 2017, from 398 the previous year. Exposure to extreme heat or drowning in irrigation canals causes many of the deaths. With the expansion of the Border Patrol pushing immigrants further into remote territory, forensic researchers say the number of deaths may be much higher. As the search began, temperatures were approaching 100 degrees, and wildfires scorched the desert nearby.