WASHINGTON — Illegal immigration, which has sparked political and social turmoil in communities across the nation, is on the wane, according to an independent report released Thursday.

The number of illegal immigrants entering the United States has slowed significantly the past few years, falling below the number of those entering the country legally, according to the report by the Pew Hispanic Center, a Washington think tank.

The report estimates there were 11.9 million illegal immigrants in the U.S. as of March. That would be a decline of 500,000 from the center's estimate a year ago. However, the change was not statistically significant because of the large margins of error.

The Pew study does not address why the decrease occurred, but other researchers cite the nation's struggling economy and stepped up enforcement of immigration laws.

"The decline in job prospects in construction, service and other low-skilled jobs are communicated through extended networks of would-be movers from Mexico and Latin America," said William Frey, a demographer at the Brookings Institution, another Washington think tank. "It also may propel more return migration."

Census data released last month showed that overall immigration slowed dramatically in 2007, though the Census Bureau does not distinguish between legal and illegal immigrants.

Illegal immigrants are notoriously difficult to count. Many researchers, including the federal government, estimate there are about 12 million illegal immigrants in the U.S.

That's a big increase from the start of the decade, when the Pew Hispanic Center estimated there were about 8.5 million.

From 2000 to 2004, about 800,000 illegal immigrants a year entered the U.S., the Pew report estimates. Since then, the average has dropped to about 500,000 a year.

A decade ago, the number of newly arrived illegal immigrants began to outnumber those legally entering the country, said the report, written by the Pew Hispanic Center's senior demographer, Jeffrey Passel, and senior writer, D'Vera Cohn.

"The reverse now appears to be true," the report said.

Illegal immigrants make up about 30 percent of all immigrants, according to the report. About four in five come from Latin America, with most coming from Mexico.

Congress has passed several measures designed to increase border enforcement, and the Bush administration has stepped up raids on businesses. Some local communities have also passed ordinances to address the issue.

Congress, however, has failed to pass a comprehensive package addressing illegal immigration, despite several attempts.

Illegal immigration has not been a big issue in this year's presidential election in part because both of the major parties' nominees, Republican John McCain and Democrat Barack Obama, support comprehensive immigration packages that include increased enforcement and an eventual path to citizenship for many illegal immigrants.

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Pew Hispanic Center: pewhispanic.org/