Mexico has its own immigration problem: American retirees

Mexico has its own immigration problem: American retirees
(Washington Post)

While figures are hard to come by, some argue that U.S. citizens may make up the vast majority of illegal immigrants in Mexico. 

That number could be as high as 1 million.

http://www.washingtonpost.com/blogs/worldviews/wp/2014/11/21/mexico-has-its-own-immigration-problem-american-retirees/