Washington ? The number of illegal immigrants living in the U.S. 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.

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.