text:  Tam, thanks so much. You're welcome. That's NPR's Tamara Keith. The number of migrants who have died attempting to cross the U.S.-Mexico border has increased over the past few years. U.S. officials say extreme summer heat is a contributing factor. The fire department in the small border city of Sunland Park, New Mexico, says it's overwhelmed with calls to rescue migrants. NPR's Sergio Martinez Beltran spent two days with the firefighters to bring us this story.