WASHINGTON — The number of illegal immigrants in the U.S. has dropped for the first time in 20 years as substantially fewer undocumented workers from Mexico, Latin America and elsewhere are crossing the border in search of jobs, an independent research group says.

The analysis by the Pew Hispanic Center suggests the nation's economic downturn and increased border enforcement have reduced the number of illegal immigrants, who make up roughly 4 percent of the U.S. population.

The findings come amid bitter debate over Arizona's strict new immigration law — now being challenged in federal court after lawmakers passed it earlier this year.

The study released today 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, before Arizona intervened with its new enforcement measures.

The study, based on an analysis of 2009 census data, puts the number of illegal immigrants about where it was in 2005.

The 11.1 million is slightly higher than the Homeland Security Department's own estimate of 10.8 million. The government uses a different census survey that makes some year-to-year comparisons difficult.