CHICAGO -- Investigators from the Centers for Disease Control and Prevention has been following a trail of mass death for the last three months through the brick and concrete valleys of this city.
The team has been trying to determine why at least 500 people, perhaps more than 700, died in a short but brutal invasion of high heat and humidity in July, creating one of the deadliest periods in the history of a city that has stubbornly survived snowstorms, fires, gangsters and skyscraper-bending winds.
"It was an extraordinary event," Dr. Joel Selanikio, a member of the federal team, said of the heat wave and its victims. "Routinely, there are very hot summers in Chicago, but you don't have 600 people die. We're trying to find out what made this distinct."
Indeed, health experts say, the staggering number of deaths is finally drawing the nation's attention to their long-ignored warnings: Heat is a public-health hazard that is deadlier than most people realize, and many cities are inadequately prepared to fight it.
The centers' investigators estimate that heat has killed more than 5,000 Americans in the past 15 years, and they expect that figure to rise as they re-examine deaths originally attributed to natural causes.
In the past three months, Chicago has been transformed into a sprawling laboratory for the study of this health threat. Investigators examining the heat-related deaths in that period -- now put at 536 to 733 -- are focusing on several areas where the city's emergency system may have fallen short.
The criteria of a heat-related death include a body temperature of 105 degrees, or more than the temperature of the residence in which the person is found.
Until the deaths began to mount, Chicago had a heat plan that was 1 1/2 pages long. It had air-conditioned shelters but no way to get vulnerable residents there. Emergency rooms in the working-class and struggling neighborhoods of the South Side were overwhelmed far sooner than those elsewhere.
Whatever the inquiry finds, one thing is clear to health experts: In the words of Dr. Thomas Houston, director of preventive medicine and environmental health for the American Medical Association, "Big cities ought to be looking at heat as a potential natural disaster. What happened in Chicago could happen anywhere."
It already has. No other city in recent memory has suffered as many heat-related deaths over such a period of time, but in 1980, 1,265 deaths around the nation were attributed by health officials to a "heat storm" that took the lives of dozens of people in St. Louis and Kansas City, Mo.
In 1993, heat killed more than 100 people in Philadelphia. Five years earlier, it took at least 60 lives in Chicago.
"It's happened before, here and in other cities," said Jennifer Neary, executive director of Metro Seniors in Action, an advocacy group. "A city the size of Chicago should have been better prepared."
Many agree, and now the National Weather Service, an Illinois Senate committee and the city are all investigating whether deaths could have been avoided with better planning and policies -- or perhaps simply more awareness.
In the early days of this year's heat wave, many in this cynical city, including Mayor Richard Daley, denied the medical examiner's numbers. But as the death count grew, the doubts melted and the finger-pointing began.
"No one is blaming the city administration for the heat," said Robert Raica, a Chicago paramedic and the Republican chairman of the state Senate's Public Health and Welfare Committee, which is conducting its own investigation. "But the city got caught off guard. The mayor's office originally laughed about the heat."
Just before the number of dead began growing, Mr. Daley held a news conference to warn his city about the weather, but not to "blow it out of proportion."