LOS ANGELES -- At a time when illegal immigration has returned to the political spotlight, figures released Wednesday show a sharp decline in the number of undocumented migrants crossing the U.S. border, in what researchers are calling the "first significant reversal" in 20 years.

The total number of illegal immigrants living in the United States dropped to 11.1 million in 2009, down from a peak of 12 million in 2007, according to estimates by the Pew Hispanic Center, a nonpartisan Washington-based group that studies the nation's Latino population. The report echoes findings of a study the Department of Homeland Security released in February.

About 300,000 illegal immigrants entered the United States each year from 2007 to 2009, down from roughly 850,000 who entered annually from 2000 to 2005, according to the Pew report.

The number of illegal immigrants coming from Mexico has remained steady over the last several years, but there has been a significant drop in illegal immigrants from the Caribbean, Central America and South America, the study found.

California still holds the largest concentration of illegal immigrants in the nation with 2.6 million, but has seen a steady decrease in this population in recent years, as job-seeking migrants flocked to Arizona, Nevada, Utah, Florida and Virginia. But those states are also reporting similar declines in their illegal migrant populations, according to the report.

The Pew study is based on extrapolations of census data and government labor statistics through March 2009.

Experts say there are a various reasons for the slowdown, including the recession and increased border enforcement.

The economy is the biggest driver for illegal immigration, said Frank D. Bean, director of the University of California, Irvine's Center for Research on Immigration, Population and Public Policy. In the economic downturn, jobs are harder to come by, particularly in construction, Dr. Bean said. "Those jobs have disappeared and have mostly stayed gone," he said.

But Dr. Bean said he did not expect the downward trend to continue once the economy rebounds. "The same reason for illegal immigrants to come to the U.S. has always been there: the need for work," he said. "As soon as work is available again, people will start coming again."

The Obama administration cited the study's figures as evidence that its efforts to strengthen border security are working. The government has cracked down on employers who hire illegal immigrants, and recently 1,200 National Guard troops were deployed along the U.S.-Mexico border to deter unlawful entry.

"The administration's unprecedented commitment of manpower, technology and infrastructure to the Southwest border has been a major factor in this dramatic drop in illegal crossings," Homeland Security Department spokesman Matt Chandler said in a statement.

The U.S. government has also stepped up removal of illegal immigrants, to 387,790 in fiscal 2009 from 291,060 in 2007.

Immigration control advocates said the decline in migrants illegally entering the United States proved that tougher enforcement works. "What this points to is that the illegal immigration population is not some unstoppable phenomenon," said Mark Krikorian, executive director of the Washington-based Center for Immigration Studies, which backs immigration restrictions. "Illegal immigrants are getting the message that the party may be over."

Mr. Krikorian said the slowdown was evidence that illegal immigration can be reduced by restricting job opportunities, and that legalizing the undocumented population was not necessary to solve the problem. The prospect of an amnesty program for illegal immigrants in the United States has long been a flash point in the heated debate over comprehensive immigration reform.

"You don't get rid of illegal immigration in one year," Mr. Krikorian said. "You get it to start shrinking by making it hard to stick around. You make it as difficult as possible to get a job here and make it as hard as possible to make a living here as an illegal immigrant."

Although some observers point to the Pew study as evidence that illegal immigrants are choosing to return to their home countries, the study's researchers said they saw no evidence that people were leaving the United States, and that the decreased illegal immigration figures stemmed from less migration into the country.

As for those migrants coming from Central and South America, UC Irvine's Dr. Bean said increased violence in Mexico due to the drug war is making it increasingly dangerous to cross through Mexico, and could possibly deter some immigrants from making the journey.

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.