Over 300 deaths suspected to be linked to this summer’s record-breaking heat wave are being investigated in a single Arizona County, official statistics showed Thursday morning.

A heat report dashboard run by Maricopa County, which includes Phoenix and is the fourth most-populated county in the United States with 4.4 million people, showed 322 deaths are suspected to have been a result of heat-related illness in the year to July 13 — a huge increase on last year.

Almost 100 of those suspected heat deaths are attributed to a single week, from July 7 to 13, when temperatures reached 118 degrees. This came after parts of Arizona, Nevada and Texas experienced their hottest June on record.

There have been 23 confirmed deaths in the county related to heat this year, the data shows, 17 of which were directly caused by heat and 6 which were “heat-contributed.”

“We know that June was hotter than June last year,” Maricopa County Public Health’s Assistant Medical Director Nick Staab told NBC affiliate KPNX of Phoenix. “We know that the risk for heat related death increases with those higher temperatures,” he said.

There have been at least 73 heat-related deaths across the U.S. this year so far, according to an ongoing count by NBC News.

And there is still no end to the extreme heat in the Southwest, with temperatures expected to reach 116 degrees in Phoenix on Friday, prompting an excessive heat warning until Saturday night. There are even the chance of some thunderstorms Thursday and through to the end of the week.

This compares to 12 confirmed heat-related deaths and 106 under investigation at this time last year, KPNX reported.

The county’s data shows that 70% of those to have died are male, while drugs were involved in just over half of the total deaths and 39% of victims were unhoused. Alcohol was involved in or contributed to 13% of deaths.

“An Excessive Heat Warning means that a period of very hot temperatures, even by local standards, will occur. Actions should be taken to lessen the impact of the extreme heat,” the National Weather Service said.

Maricopa residents can call 211 to find out their nearest cooling center and request free transport. Firefighters in Phoenix have, this year, pioneered the use of ice immersion bags to aggressively tackle heatstroke and bring patients’ body temperatures down.

Maricopa’s data is broken down into zip codes and the area with the most heat-related deaths is the city of Mesa, east of Phoenix.

Last year there were 579 confirmed and 56 suspected deaths related to heat in Maricopa, compared to 386 in 2022, according to data from the Department of Heath and Human Services.

The DHH said a quarter of last year’s deaths happened indoors, with air conditioning units not working properly for two thirds of those victims.

The heat dashboard was launched by the county in May to better track illness and death as a result of extreme temperatures.

NBC News reached out to Maricopa County for comment.