PHOENIX - The number of illegal immigrants in Arizona is down much more sharply than in the rest of the country, a new estimate says.

New figures from the Pew Hispanic Center say that in 2009 there were about 375,000 people in Arizona who either entered the country illegally or overstayed their legal visas. That compares with Pew's estimate 475,000 for 2008. That 21 percent drop compares with a decline of just 4.3 percent nationwide.

The new figures, released Wednesday, show a sharp departure from the trends Pew has been tracking since 1990: This is the first time the numbers have dropped.

Jeffrey Passel, senior demographer for the organization, which studies Hispanic population and employment trends, said states with healthier economies seem to have been less affected. For example, the estimates of the "unauthorized" population of Texas between 2007 and 2008 are, within margins of error, virtually unchanged.

But Passel said he could not explain the sharp drop in Arizona. He said it is impossible to determine whether the change is the result of a law approved in 2007 that allows judges to punish employers found guilty of knowingly hiring illegal immigrants.

Pew researchers said the numbers, based on estimates from March 2008 Census Bureau figures, don't reflect more recent developments such as the legislative debate and gubernatorial approval of SB 1070. That law includes several provisions designed to let state and local police arrest illegal immigrants.

Several of those provisions have been put on hold by a federal judge. But the judge allowed other sections to take effect on July 29, including a new law aimed at day laborers and a prohibition against public officials from limiting their police officers from enforcing federal immigration laws.

Passel said it may be difficult to separate the effects of the economy from factors such as new state laws or even stepped-up border enforcement.

"As somebody who's been doing estimates here for a long time, I have observed that the debates about illegal immigration get more heated when the economy is in trouble," he said.

That, however, did not stop the Obama administration, under fire from some quarters for failing to secure the border, from claiming responsibility for the change.

"This 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," said Matt Chandler, spokesman for the U.S. Department of Homeland Security.

In a prepared statement, Chandler said his agency, headed by former Arizona Gov. Janet Napolitano, is cracking down on employers who hire illegal immigrants and that deportations, at least of illegal immigrants who have prior criminal records, "are at an all-time high."

There is no official count of illegal immigrants.

While the Census Bureau, in both its decennial count and its annual surveys, asks for information such as place of birth, the agency does not inquire about legal status. But Passel said there are ways to extrapolate some numbers based on numbers of people who are legally admitted to this country, compared with the number who say they were born outside the United States and other adjustments.

So what happened to those migrants?

Within the country, some likely followed the jobs.

"They may well be going to different parts of the country," Passel said. He said that could be why Texas, where the economy is better, is not showing the same declines as other states.

On a nationwide basis, Passel stressed, this is not a "static population." He said that while there always have been illegal immigrants coming into the country, there also are people who leave. Others already here gain legal status. And some smaller percentage simply die.

Within the new report there are some identifiable trends.

Passel said most of the decline in new migrants was among those from Latin American countries other than Mexico. During the last two years, he said, migration from those countries has dropped by 22 percent.

Mexican migrants represent about 60 percent of the total nationwide, a number that is likely larger in border states such as Arizona.

"It's a very young population," Passel said. He estimated that 58 percent of illegal immigrants are between 18 and 39; for the U.S.-born population, that figure is just 28 percent. And men outnumber women by a 3-to-2 margin.