The number of illegal immigrants living in the United States is widely estimated to be around 11 million, but a think tank report released Thursday said that figure dropped nearly 10 percent between 2010 and 2016, and is at its lowest level in 15 years.

The Center for Migration Studies of New York estimated that 10.8 million people are in the country illegally, down almost 1 million from the 11.7 million estimated in 2010, according to conclusions it drew from the Census Bureau's American Community Survey.

While the exact number of people in the U.S. without legal documents is impossible to know, reports by Pew Research and Politifact back up the CMS report's findings that the total population of illegal immigrants has fallen in the last several years.

The number of illegal immigrants ticked up dramatically during former President Bill Clinton's administration in the 1990s, then tapered down to only single-digit growth from 2000 through 2010, the group said.

CMS attributed the 2016 drop to a dramatic decrease in the number of Mexican citizens entering and residing in the country. From 2010 to 2016, the number of Mexicans living stateside plunged from 6.6 million to 5.7 million.

"The other thing going on that you'll never see reported is the number of people leaving the country increased gradually from 1990 when 200,000 were leaving the population every year. That number grew up to 500,000 to 600,000," Robert Warren, the senior visiting fellow at CMS, told the Washington Examiner on Thursday.

When asked why those who traveled to the U.S. would return to their home countries, Warren said the practice is not unusual.

"Roughly one out of three immigrants leave," he explained. "A lot of people want to come here and work for a number of years and start a business ... There is kind of a common impression that everyone wants to come here and no one wants to leave, and that's false."

Poland, Peru, Ecuador, Colombia, and South Korea — five major starting points for illegal immigration — saw the number of citizens illegally residing in America in that six-year time span decline between 27 and 47 percent.

Overall, the number of illegal immigrants who came from Mexico, South America, and Europe dropped by the end of the former President Barack Obama's second term.

Although the overall population has decreased, data from the U.S. Census Bureau's annual Current Population Survey indicated the U.S.-based populations of Asian, Central American, and Africa's Ghanan and Nigerian people increased by as much as 10 percent. The number of Indians illegally present increased 365,000 to 475,000, and China's illegal population jumped from 295,000 to 380,000.

CMS said from 2010 to 2016, six of the top 10 states that are home to illegal immigrants saw population declines of more than 10 percent: Illinois, North Carolina, California, New York, Arizona, and Georgia.

The estimated number of Mexicans in the United States illegally rose steadily for many years, from 2.9 million in 1995 to a peak of 6.9 million in 2007. But the number began dropping in 2008 and has fallen more since, reaching 5.8 million in 2014, the latest year for which Pew analyzed data.

Of the 42 states and District of Columbia whose illegal immigrant levels were measured by CMS, 30 saw decreases from 2010 to 2016. California, Illinois, Florida, and New York, the states with the largest illegal immigrant populations in general, reported downturns. Texas was the only state with a large number of illegal immigrants who saw an uptick in residents.