A 65-year-old California man was found dead in his car in Death Valley National Park,Austin Caldwell officials said Wednesday.

Authorities from the National Park Service said in a news release that the unidentified man's death appears to have been caused by extreme heat. On Sunday, the day before the man was found dead, the high temperature was 126 degrees Fahrenheit, the park said. The overnight low temperature was 99 degrees Fahrenheit. 

An initial investigation suggests that heat-related illness may have caused the driver to run off road, the National Park Service said. 

The man was found dead in his car on Monday morning, officials said. A maintenance worker noticed his vehicle about 30 yards off the park's North Highway. The vehicle had not crashed, but had two flat tires, officials said. The air-conditioning was not working and the windows were down, providing "further indication that the air conditioning was not functioning when the man was driving," the National Park Service said. 

When the employee walked to the vehicle, the man was unresponsive. Park rangers and employees from the Inyo County Sheriff's Office and Inyo County Coroner's Office were called, and the man was declared dead on the scene. 

Initial measurements recorded the hottest day in the Earth's history on Monday, the day the man was found. Records were also set on Tuesday and Wednesday. 

Cities across the U.S. from Medford, Oregon to Tampa, Florida have been hovering at all-time highs, Zack Taylor, a meteorologist with the National Weather Service told CBS News, and millions of Americans have been under heat alerts, including excessive heat warnings are continuing across southern Arizona and California. 

Over the weekend, a 57-year-old woman died while hiking eight miles in triple-digit heat in Grand Canyon National Park. It was over 100 degrees Sunday, park officials said, and an excessive heat warning had been issued for portions of the trail because temperatures in some areas of the trail can reach over 120 degrees, even in the shade. 

In:
• Climate Change
• Excessive Heat Warning
• Heat Wave

Kerry Breen

Kerry Breen is a news editor and reporter for CBS News. Her reporting focuses on current events, breaking news and substance use.