WASHINGTON (Reuters) - The massive heat wave that baked half the United States in triple-digit heat indexes last week may have caused as many as 64 deaths in 15 states, the National Weather Service said late on Tuesday. Forecasters said the land area still suffering from the excessive heat has diminished significantly, but some regions could suffer from a return next week. "The high pressure system responsible for the recent heat wave will remain in place across the central US through Thursday, then a brief reprieve is in store," a National Weather Service statement said. "The long-range forecast has heat rebuilding into the Central and Southern US by early next week," the forecast projected.