Some researchers said that the drop is because fewer illegal immigrants were entering the country.

But others disagree, saying the decline is largely due to undocumented immigrants either voluntarily leaving or being deported.

Most researchers said that the downturn in the economy has played a key role in reducing the illegal immigrant population. But some point out that the drop in the undocumented population started before the recession.

The number of illegal immigrants residing in the U.S. declined to 11.6 million in 2008, from 11.7 million the previous year, according to the Department of Homeland Security. To see the full report, visit http://www.dhs.gov/files/statistics/immigration.shtm.