"High heat like this can pose real threats to your health," the national park's superintendent said in a statement. "While this is a very exciting time to experience potential world record-setting temperatures in Death Valley, we encourage visitors to choose their activities carefully, avoiding prolonged periods of time outside of an air-conditioned vehicle or building when temperatures are this high." The statement warned that the heavy safety gear necessary for motorcycle riding adds to the danger, and that heat illness and injury can build up over the course of multiple days and have a cumulative effect. The group was traveling through Badwater Basin, an area that includes North America's lowest point at 282 feet below sea level, per NPS. Death Valley lies mostly in California, but portions of it are in Nevada.

A long-running heat wave that has already shattered previous records across the U.S. persisted on Sunday, baking parts of the West with dangerous temperatures that caused the death of a motorcyclist in Death Valley and holding the East in its hot and humid grip. An excessive heat warning — the National Weather Service's highest alert — was in effect for about 36 million people, or about 10% of the population, said NWS meteorologist Bryan Jackson. Dozens of locations in the West and Pacific Northwest tied or broke previous heat records.

The temperature at the vast park was 128 degrees on Saturday. Because of that scorcher, a helicopter was unable to fly to reach the motorcyclists and transport them to hospitals. “While this is a very exciting time to experience potential world record setting temperatures in Death Valley, we encourage visitors to choose their activities carefully, avoiding prolonged periods of time outside of an air-conditioned vehicle or building when temperatures are this high,” park Superintendent Mike Reynolds said.