 In all three cases, bystanders and first responders attempted CPR on the hikers, but were unable to resuscitate them. The National Park Service has warned temperatures on the trails can reach 120 degrees. It advised visitors to avoid hiking in the inner canyon from 10 a.m. to 4 p.m. and notes emergency responders may also take longer in the summer due to understaffing. Ayanna Archie, NPR News.