After a string of scorching days in June 2023, the body of an 88-year-old man was discovered in his home in Maricopa County, Ariz. His air-conditioner, set to 70, was blowing hot air. The temperature inside was nearly 110 degrees. Maybe he had heart problems. Maybe a different organ broke down. Maybe he was taking medications that did not work as they should. Did extreme heat cause or contribute to his death?

After further investigation, the answer was yes, officials said. But scenarios like this can play out in any heat wave across the country, and in many cases, the answer cannot be determined immediately — if at all. That dearth of information is the reason that heat-related deaths in the United States remain a frustrating mystery.

Researchers estimate that heat kills more people than any other extreme weather event, and the number of heat-related deaths reported by the U.S. Centers for Disease Control and Prevention has steadily risen in recent years.

In 2023, the agency reported that heat played a role in approximately 2,300 deaths, though this number may be revised as more records are processed. But some researchers say the actual number is far higher. One study that examined data from the late 1990s to the early 2000s concluded that the average number of fatalities annually was roughly 10,000.

The C.D.C. relies on death certificates reported by local authorities for its tally, but the way these certificates are completed varies from place to place. Many local officials do not have the time, funding or staffing needed to investigate heat-related questions. And officials do not apply a consistent set of criteria to determine whether heat contributed to a death — or even consider heat as a potential factor when filling out death certificates.