(CNN) -- The undocumented immigrant population in the United States rose to an estimated 11.7 million in 2012, up from 11.5 million in 2011, according to report released by the Pew Research Center Hispanic Trends Project.
The number includes those who crossed U.S. borders illegally, as well as those who came in with temporary visas but stayed past their expiration date, Pew said Monday about its estimate, based on U.S. government data.
Mark Hugo Lopez, director of Hispanic research at Pew and co-author of the study, says the data available cannot point to a particular cause for the increase, but he says the improving U.S. economy could be a factor.
According to Pew, in 2007, there were an estimated 12.2 million undocumented immigrants in the United Sates. The number decreased to 11.3 million by 2009, breaking a rising trend that had held for two decades. One reason for the decrease could have been the recession.
In 2009, the Center for Immigration Studies, a Washington-based think tank that seeks a "pro-immigrant, low-immigration vision," said Census data indicated that more than 1 million undocumented immigrants left the previous year, a departure that began even before the nation's economy took a turn for the worse.
Immigrants from Mexico are still the largest group within the undocumented population -- 52% as of 2012 -- but the number of recent Mexican immigrants has decreased at higher rates than the general group. That signals a shift in the demographic makeup of those entering illegally into the United States.
"They are coming from all parts of the world other than Mexico. Central American immigrants are a part of that, but so are immigrants from Asia and Europe," Lopez said. "The report points to border apprehension trends, too -- the number of Mexicans apprehended at the border by the border patrol is down in the last year or so."
Pew estimated there are now about 6 million Mexican undocumented immigrants in the United States.
Lopez said that Southern states such as Georgia and North Carolina have seen an increase in undocumented immigrants because their economies have fared better than California since the 1990s.
Texas, a state that weathered the recession better than others, was the only state that didn't see a drop in undocumented immigrants in the past few years.
According to Pew, its methodology "compares a demographic estimate of the number of immigrants residing legally in the country with the total number of immigrants as measured by a survey." The difference is assumed to be the number of unauthorized immigrants in the survey, a number that is later adjusted for omissions from the survey.
The new data could also affect the debate on a potential comprehensive immigration reform which has stalled in the House of Representatives after the Senate approved a bill earlier this year that would provide a path to citizenship to those living here illegally.
CNN's Cindy Y. Rodriguez contributed to this report.