WASHINGTON (AP) — The number of Mexican immigrants in the U.S. illegally has declined so sharply over the past decade that for the first time, they no longer make up the majority of that category, according to an estimate by the Pew Research Center Wednesday.

But the number of Central Americans in the country illegally is increasing — from 1.5 million in 2007 to 1.9 million in 2017, the study found.

Pew, like other researchers, cited declining Mexican birth rates as a major reason. In 2015, Pew reported that more Mexicans were leaving the U.S. than arriving, another milestone marking the end of one of the greatest waves of immigration in U.S. history, dating back to 1965 and ending around the time of the Great Recession.

The numbers reflect the conundrum the U.S. is facing at the southern border: The number of Central American migrants crossing the U.S.-Mexico border is rising dramatically, and they are not easily returned over the border — unlike in previous years, when the majority of the border crossers were single men from Mexico.

Curbing immigration is President Donald Trump’s signature political issue, but his hard-line and chaotic border policies have failed to stem the tide. In fact, the numbers have increased since he took office.

There were about 4.9 million Mexicans in the U.S. illegally in 2017, down 2 million from 2007. The decrease was the major driver in bringing down the overall population of immigrants in the country illegally. In 2017 it was about 10.5 million — the lowest since 2004. The research group found the peak was in 2007 at about 12.2 million. Previously, Mexican nationals made up most of that population. Now, it’s a combination, with Central America having the second-largest, and Asia following with 1.4 million.

The highly regarded Pew estimates track trends in U.S. government data. Mexicans made up 55 percent of an estimated 12 million people in the U.S. illegally in January 2015, according to the latest Department of Homeland Security data, compared with 55 percent of an estimated 11.6 million people five years earlier.