A heat wave that spread from the Midwest to the Northeast tormented millions of people with blasts of 100-degree temperatures and bog-like humidity as blackouts struck neighborhoods and deaths were blamed on the hot weather.

There was little hope that Saturday would bring much relief until the evening, with the National Weather Service warning of excessive heat in several states, including parts of Oklahoma, Indiana, New York, New Jersey and Connecticut. It predicted “oppressive heat” with temperatures at least in the 90s.