When the Olympic Games came to Atlanta in 1996, a building boom transformed the landscape of downtown and brought with it an influx of Latino immigrants _ both legal and illegal.

In the years since, the number of illegal immigrants living in Georgia has skyrocketed, more than doubling to 480,000 from January 2000 to January 2009, according to a new federal report. That gave Georgia the greatest percentage increase among the 10 states with the biggest illegal immigrant populations during those years. Many in metro Atlanta say the explanation for the boom is simple.

"It was because of jobs," said Kathy Brannon, who worked for the suburban city of Chamblee for nearly 30 years. "That's why people have come to this country since it started, for opportunity."

For years, Chamblee was the last stop for three bus companies carrying immigrants from the border city of Brownsville, Texas, said Brannon, the retired city manager. With cheap housing, easy transportation and an abundance of work, the immigrants put down roots and were quick to tell family and friends back home of the opportunities in the Atlanta area.

To get a better sense of how much the illegal immigrant population has grown in Georgia, consider that the state had just 35,000 of them in 1990, according to estimates by the Pew Hispanic Center.

Demographer William Frey of the Brookings Institution said illegal immigrants moved where they could find work in low-skilled fields like construction and the service industry, which were booming across the Sun Belt states along with higher-skilled jobs.

"In a way it could be a sort of badge of success to have a higher undocumented immigrant population" because it means the economy is strong, Frey said.

North Carolina, another fast-growing Southeastern state during those years, is also one of the top 10 states for the sheer size of its illegal immigrant population, estimated at about 370,000 in January 2009 as compared to 260,000 in 2000, according to the report by the Department of Homeland Security's Office of Immigration Statistics. The agency relied on data from the American Community Survey, a nationwide sampling conducted by the U.S. Census Bureau.

The large immigrant populations in Georgia and North Carolina are largely Mexican and undocumented, said Jeff Passel, a senior demographer at the Pew Hispanic Center.

As recently as the 1980s, Southeastern states _ with the exception of Florida _ had very few immigrants, legal or illegal, Passel said. California, which is still home to about 24 percent of the country's illegal immigrants, used to account for about 40 percent. Five other states _ Texas, New York, Florida, Illinois and New Jersey _ shared another 40 percent, he said.

But a recession in California in the early 1990s, and a ready supply of low-skilled jobs in other regions prompted immigrants to look elsewhere, especially the Southeast, Passel said.

Immigrants are vital to the economy in the Southeast, especially the agriculture, construction and service industries, said Jerry Gonzalez, executive director of the Georgia Association of Latino Elected Officials.

"It started with the Olympics. Atlanta would not have been able to finish the construction in time for the Olympics without immigrant labor, and specifically Mexican immigrant labor," he said. "After that, the housing boom that the Southeast experienced, and specifically Georgia, would not have been possible without immigrant labor."

The impact of illegal immigrants is tough to measure because they generally keep a low profile. But Passel said they are drawn by jobs, so most are employed and have income taxes and social security payments withheld from their paychecks.

While illegal immigrants are not eligible for welfare and many other public benefits, their U.S.-born children are, but they tend to underuse those services, Passel said.

Critics point to hospital emergency rooms, which must treat everybody regardless of their ability to pay, and public schools as places where illegal immigrants are a burden on local communities. Some also blame illegal immigrants for crime and driving down wages for low-skill work.

Nationwide, the report found that the illegal immigrant population grew 27 percent during the study period, though the numbers fell in the last two years. The population was 11.8 million in January 2007. It fell to 11.6 million in January 2008 and dropped to 10.8 million in January 2009. That coincides with the downturn in the U.S. economy, and demographers say the drop is likely to be temporary.

"If you look back over the last 20 years, the inflow of undocumented immigrants goes up and down with the U.S. economy," Passel said.

A rough economy hits illegal immigrants even harder than citizens and legal immigrants, he said. But once the economy rebounds, construction will pick up, as will the service industry, and illegal immigrants will return for those jobs.

Demographers expect the Southeast to bounce back faster than states like California, Nevada and Arizona. And they don't expect hostile attitudes or get-tough laws to keep illegal immigrants from coming back to Georgia.

"The only way you're going to get the illegal immigrant population in Georgia to go down is to legalize them or get rid of the jobs," said Dowell Myers, a specialist in demographic trends at the University of Southern California.