By Hope Yen The Associated Press

The number of illegal immigrants living in the United States has dropped for the first time in two decades - decreasing by 8 percent since 2007, a new study finds. The reasons range from the sour economy to Mexican violence and increased U.S. enforcement that has made it harder to sneak across the border.

In California, the decline from 2008 to 2009 was 4 percent, but the state is still home to nearly a quarter of the nation's population of illegal immigrants.

Much of the decline comes from a sharp drop-off in illegal immigrants from the Caribbean, Central America and South America attempting to cross the southern border of the U.S., according to the Pew Hispanic Center, which based its report on an analysis of 2009 census data.

The findings come amid bitter debate over Arizona's strict new immigration law, which was passed earlier this year but is on hold for now as it is challenged in federal court. The Obama administration contends the state law usurps federal authority and promotes racial profiling, while Arizona leaders say states are justified to step in if federal enforcement falls substantially short.

The study released Wednesday estimates that 11.1 million illegal immigrants lived in the U.S. in 2009. That represents a decrease of roughly 1 million, or 8 percent, from a peak of 12 million in 2007.

The study puts the number of illegal immigrants down to about where it was in 2005. They still make up roughly 4 percent of the U.S. population.

The Homeland Security Department's own estimate of illegal immigrants is slightly lower, at 10.8 million. The government uses a different census survey that makes some year-to-year comparisons difficult.

An increase in unauthorized immigrants leaving the U.S., by deportation or for economic reasons, may have played a factor in the falling number.

In recent years, U.S. Immigration and Customs Enforcement has deported an increasing number of illegal immigrants, reaching a high last year of more than 389,000 people, according to government figures.

States in the Southeast and Southwest saw some of the biggest declines in the number of illegal immigrants from 2008 to 2009, including Florida, Nevada and Virginia. Arizona saw a decrease, but it was too small to be statistically significant.

It's hard to figure out how much of the decline to attribute to the bad economy and how much to federal immigration enforcement, said Jeffrey Passel, a senior demographer at Pew who co-wrote the analysis.

"They're certainly acting together," he said. Passel said illegal immigrants now find it more expensive and dangerous to cross into the U.S. and also have less incentive to do so given the languishing job market in construction and other low-wage industries.

The Pew analysis found the sharpest decline - 22 percent - among illegal immigrants from Latin American countries other than Mexico. This may reflect a lesser-known aspect of the immigration debate that broke into the news again last week when the bodies of 72 Central and South American migrants were found in a cartel stronghold in northern Mexico: Thousands of non-Mexicans go missing on their way to the United States, falling victim to demands that they pay impossible ransoms to criminal gangs that control access to the U.S. border.

"While people are arguing the government is not stopping illegal immigration, our data suggests the flow of undocumented immigrants sneaking into the country has dropped dramatically," Passel said.

The estimates by Pew will add to the political back-and-forth on immigration reform.

President Barack Obama, who is challenging the Arizona law, has pledged to push an overhaul of federal immigration law but has declined to set a timeline.

After the passage of Arizona's immigration law, more than a dozen states were considering similar legislation or have issued legal opinions aimed at strengthening immigration enforcement. They include Florida, Virginia, South Carolina and Utah. 

Boosted by immigration and high numbers of births among Latinos, minorities now make up roughly half the children born in the U.S., part of a historic trend in which they are projected to become the majority of Americans by midcentury. Roughly one in four counties currently has more minority children than white children or is nearing that point.

The Pew analysis, based on census data through March 2009, estimates California had 2.55 million illegal immigrants that year, a 3.8 percent decrease from the year before. The state accounts for 23 percent of illegal immigrants in the United States.

The study also says illegal immigrants now comprise 6.9 percent of California's population - the highest percentage in the country.

Despite the decline, Los Angeles County officials said they have seen an increase in the costs of providing services to undocumented immigrants and in the number of citizen children of undocumented parents receiving food stamps and welfare benefits.

The amount of money the county spends on health, welfare and criminal justice services for undocumented immigrants increased from $1 billion in 2008 to $1.4 billion in 2009, according to figures provided by county Supervisor Mike Antonovich. By including the education costs for the children of undocumented immigrants, said Antonovich spokesman Tony Bell, said the annual costs now exceed $2 billion.

"It's been increasing gradually, but consistently over the years," Bell said. "It leads me to believe that illegal immigration continues to have a catastrophic financial impact on county taxpayers without regards to surveys showing a smaller population or not."

From January to July, the number of citizen children of undocumented parents receiving food stamps in the county increased 4 percent from 103,238 to 107,504, according to the Department of Public Social Services.