Phoenix-area paramedic waits to transport a resident to the hospital during extreme heat on July 20.
CNN  — 

Extreme heat in the US has killed at least 147 people in just five counties, medical examiners report — a mere snapshot of the fatal toll this searing summer is taking. And experts say that estimate is likely far lower than the actual number of lives lost to the excruciating heat.

The deaths reported here occurred in three states that have endured the worst of this summer’s vicious heat: As of early August, 64 had died in Pima County, Arizona; 39 in Maricopa County, Arizona; 26 in Clark County, Nevada; 11 in Webb County, Texas; and seven in Harris County, Texas.

Several heat-related deaths were also reported in California, parts of the South and the Midwest, though the tolls have not been as high as those in the five counties reported here. Maricopa County, which is home to Phoenix and has officially tallied at least 39 heat-related deaths, has 312 more deaths still under investigation for possible heat-related causes of death.