In a study published Tuesday in the journal PLOS Medicine, researchers concluded that at least 461,000 "excess" Iraqi deaths occurred in the troubled nation after the U.S.-led invasion that resulted in the overthrow of President Saddam Hussein. Those were defined as fatalities that would not have occurred in the absence of an invasion and occupation.

The study's release follows several controversial and widely varying estimates of Iraq war deaths. It is the first analysis published since 2006, the bloodiest period of the war.

Lead author Amy Hagopian, an associate professor of global health at the University of Washington, said the analysis was limited by a lack of accurate health and census reporting in Iraq. However, she said, it was a duty of public health officials to assess the effects of war.

"It's a politically loaded topic," Hagopian said. "Everyone's against polio and wants to eradicate it, but it's different with war."

According to Hagopian and her colleagues, at least 60% of the excess deaths were the result of violence. The rest were linked to so-called secondary causes.

"War causes a huge amount of chaos, disruption and havoc," Hagopian said. "Some deaths are direct, but there are also deaths that result from destroyed infrastructure, increased stress, inability to get medical care, poor water, poor access to food.... These are all reasons why people die."

Of those deaths determined to be the result of direct violence, the study attributed 35% to coalition forces, 32% to sectarian militias and 11% to criminals. Contrary to public perception of mayhem in Iraq, bombings accounted for just 12% of violent deaths. The overall majority of violent deaths, 63%, were the result of gunfire.

The task of estimating war deaths in Iraq has been extremely difficult.

When epidemiologists in developed nations study mortality, they use real-time death reporting databases or census comparisons to calculate overall rates. These methods are largely unavailable in Iraq, so researchers must rely on a less accurate method: surveying families about their deceased relatives.

For the study released Tuesday, a team of Iraqi physicians visited 2,000 randomly selected homes and asked residents to recount all family deaths that had occurred in the period beginning two years before the invasion, through 2011.

Based on the survey, researchers arrived at a crude death rate and then applied that to Iraq's estimated population of more than 32 million. That figure was then corrected for deaths that would have been reported by families who fled the nation.

"One of the problems clearly is that we're asking people to remember a very long period of time," Hagopian said. "There can be a lot of forgetting, and that forgetting will be in favor of a lower count."

Several experts who were not involved in the study said it provided a crude picture of Iraq war deaths and was therefore valuable.

However, they raised concerns about the lack of accurate census data.

"The main problem with this survey, and with others, is the underlying denominator: the total population," said Dr. Prabhat Jha, director of the Center for Global Health Research in Toronto. "That's the tricky part of war zone situations. People not only get killed, but they move out of the country and rarely move in."

To provide perspective on the war death figures, Jha said that the 1918-19 flu pandemic killed 20 million to 60 million people. During the 20th century, about 200 million people died worldwide because of war, famine and natural disaster. During that same period, about 2 billion died of childhood diseases.

Estimates of Iraq war deaths have varied greatly over the years.

In a study that appeared in the journal Lancet in 2004, Les Roberts and his colleagues estimated that about 100,000 deaths occurred during the first year of the war; a second study he helped conduct estimated that more than 600,000 deaths had occurred by 2006.

In both cases, Roberts said, officials in the George W. Bush administration dismissed the estimates as not credible. He said the latest study is further evidence that the Iraq war death toll is much higher than officials realized or chose to acknowledge.