(Reuters) – The population of unauthorized immigrants in the United States fell to 10.7 million in 2016, its lowest level since 2004, due largely to a decline in the number of people coming from Mexico, a study released on Tuesday said.

The report from the Pew Research Center pewrsr.ch/2Qptbid, based on U.S. Census data and other figures from 2016, showed the number of illegal immigrants in the United States has declined steadily since its peak of 12.2 million in 2007.

Researchers believe part of the reason for the decline was the economic recession that gripped the United States in 2007 and the slow recovery that followed, which limited work opportunities for migrants.

“The combination of economic forces and enforcement priorities may be working together to discourage people from arriving, or sending them home,” said D’Vera Cohn, one of the authors of the Pew Research Center report.

President Donald Trump has made immigration enforcement a focus for his administration, most recently pressing the U.S. Congress to authorize funding of a wall on the border with Mexico and deploying troops in advance of the arrival of a caravan of migrants from Central America.

Even before Trump took office, a decline in the number of illegal immigrants from Mexico had changed the demographic profile of unauthorized migrants in the United States.

Mexico is still the country of origin for about half the unauthorized immigrants in the United States, but their number in that total population fell by 1.5 million between 2007 and 2016, the Pew report found.

During that decade, the number of unauthorized immigrants from Central America increased by 375,000.

VISA OVERSTAYS

With the share of Mexicans decreasing, Asians account for 22 percent of unauthorized immigrants who recently arrived in the United States, the report found.

Among recent arrivals, immigrants in the United States who overstayed a visa were likely to outnumber people who illegally crossed the border, it said.

Overall, the Pew study was in line with previous research that has found many unauthorized immigrants have been living in the United States for years and their children are more likely to have been born in the country than abroad.

Among the 10.7 million unauthorized immigrants, two-thirds of adults have lived in the United States for more than a decade, the Pew Research Center study found.

Five million U.S.-born children with American citizenship are living with parents or relatives who are unauthorized immigrants, the study found.