As recently as last month, state officials reported that the two-week ordeal of triple-digit temperatures that began on July 14, 2006, killed 143 people.
But statistics compiled by the AP from each of California's 58 counties show the number of deaths last July was 466 higher than the average over the previous six years - a spike many health officials attribute to the broiling heat.
Dr. Kevin Reilly, the state Health Department's deputy director for prevention services, said that without examining each case individually, he cannot say for certain whether the 300 or so additional deaths identified by the AP were heat-related. But he admitted the state's count is probably too low.
"Is this underreported? Almost certainly," he said.
Frank McCarton, chief deputy director of the Governor's Office of Emergency Services, said state and county health officials alike clearly need to track heat-related deaths more accurately if they hope to save more lives.
"This is something we're going to really try to tighten up," he said.
That could be especially important because many scientists predict California will experience prolonged heat waves for years to come.
There is no evidence of a deliberate undercount of fatalities by California authorities.
Instead, local health officials say they often have difficulty determining whether a death was caused by heat instead of old age or a pre-existing condition, such as heart or respiratory disease - illnesses that can be aggravated by extreme temperatures. Coroners commonly attribute a death to one of these underlying conditions without noting whether heat was a factor.
California coroners typically list deaths as heat-related when there are obvious connections, such as an elderly or disabled person found dead in a stifling room or farmworkers who collapsed in the fields. Beyond that, settling on a cause of death is largely a judgment call.
Other states and cities have had similar disagreements over heat-wave death tolls, in part because there is no fixed, nationwide standard for deciding whether a death was heat-related.
For example, a 1995 heat wave in Chicago was blamed by the Cook County medical examiner's office for more than 700 deaths. Mayor Richard Daley criticized the count as too high, but the numbers ultimately were backed up by the federal Centers for Disease Control and Prevention.
California's McCarton said officials need to do a better job of investigating deaths that may be heat-related. Also, he said the state should do more to track early warning signs - a spike in ambulance calls, hospital admissions or livestock deaths - and then warn medical examiners to look more closely at whether human deaths were caused by the heat.
He and other public health officials said authorities need an accurate accounting of how many victims died, how and where in order to form an effective plan for saving people from extreme heat.
During scorching heat, California, like other states and cities, opens air-conditioned cooling centers and urges local governments to check on shut-ins and the elderly. A state emergency plan developed after last year's heat wave says social workers and National Guardsmen should take people to cooling centers.
The AP analysis found that the average number of deaths statewide for July between 2000 and 2005 was 18,639. Last July, it was 19,105, according to the AP data.
Deaths were above average in 37 of California's 58 counties. Many of the 37 were the same counties where temperatures rose into the triple digits.
In Sacramento County alone, the July 2006 death toll was more than 100 over the average for the previous six years.
"I cannot account for any event that could have caused that, other than the heat wave," county Health Officer Glennah Trochet said.
Outside experts who reviewed the data said that while some of the increase was due to California's population gain, the official toll significantly underestimated the deaths attributable to the heat.
"Those are gross under-exaggerations of what is really happening," said Laurence Kalkstein, who heads a climatology laboratory at the University of Miami and tracks heat wave deaths. "I would be quite confident that 143 is well below the actual number of heat-related deaths. In general, we see underestimates by a factor of two or three."
This summer, the National Weather Service is forecasting a 40 percent chance for hotter-than-normal temperatures in California's Central Valley.
"The pattern over the western United States is going to be very favorable for a very long, hot summer," said forecaster Jeff Barlow.
Associated Press writers Solvej Schou in Los Angeles, Marcus Wohlsen in San Francisco and Juliet Williams in Sacramento contributed to this report