The number of unauthorized immigrants illegally residing in the United States decreased in 2017 to the lowest level in roughly 15 years, largely due to the departure of millions of Mexican citizens over the past decade, according to a newly released Pew Research Center study.

Mexican citizens have historically made up the majority of the undocumented population. But Pew found Mexicans now no longer comprise the majority of illegal immigrants, with more citizens from countries other than Mexico — 5.6 million — now illegally in the states. In 2007, approximately 6.9 million Mexican citizens were known to be living in the U.S. The number dropped in 2017 to 4.9 million.