The Pew Research Center report found that as of March 2012, 11.7 million unauthorized immigrants were living and working in the shadows in the United States, according to a preliminary estimate based on U.S. government data.
The number of such immigrants in the country peaked at 12.2 million in 2007 and fell to 11.3 million in 2009, bucking an upward trend that had held for decades, the study found. The center cautioned that the new findings were provisional.
"Although it appears that the unauthorized immigrant total has begun to grow again, the data are insufficient to say so definitively," the report by the nonpartisan think-tank noted.
A more definitive count will be possible when further microdata from the U.S. Census Bureau's American Community Survey becomes available later in the year, the report said.
A landmark immigration overhaul passed by the Democratic-led U.S. Senate in June includes a path to citizenship for millions of immigrants currently living illegally in the United States and tighter borders, although it faces scant chance of passage in its current form in the Republican-controlled House of Representatives.
The study found different trends emerged in the six U.S. states in which 60 percent of unauthorized immigrants live - California, Florida, Illinois, New Jersey, New York and Texas. Of these, only Texas showed increases in its unauthorized immigrant population over the four years to 2011, but no decreases.
