The number of people in the United States illegally may be on the rise again after a significant decline during the recession, a research organization reported Monday.

The Pew Research Hispanic Trends Project estimated 11.7 million people in the country illegally - "the unauthorized immigrant population," as the report labels it - in March 2012. That was an increase from Pew's estimate of 11.2 million in 2009, when the recession ended.

"The sharp decline in the U.S. unauthorized immigrant population that accompanied the great recession has bottomed out, and the number may be rising again," the Pew report said.

It said trends differ from state to state, although Pew did not release numbers for Arizona. It said such numbers would be released in a future report.

At its peak in 2007, the number of people in Arizona illegally was estimated at 500,000. Subsequent estimates by Pew and other researchers have put it at 300,000.

"The analysis also finds that the post-2007 population dip was sharper for Mexicans than for unauthorized immigrants as a whole," the report said.

That is attributed to several factors, including the U.S. recession, a stronger Mexican economy and U.S. border enforcement.

The Pew study cautioned that the numbers it released are not definitive.

"Although it appears that the unauthorized immigrant total has begun to grow again, the data are insufficient to say so definitively," the report said. "The difference in the size of the unauthorized population from 2010 (11.4 million) to 2012 (11.7 million), or from 2011 (11.5 million) to 2012 is not statistically significant."