Federal law enforcement is making a higher share of arrests for immigration crimes than ever before, while the number of drug and firearms arrests continues to fall.

A Pew Research Center analysis of data from the Bureau of Justice Statistics shows that 50 percent of the 165,265 total arrests made by the federal government in 2014 were for immigration-related offenses.

In 2004, that figure was 28 percent.

Drug and firearms arrests, on the other hand, have dropped over the past 10 years. The share of federal arrests for narcotics crimes fell from 23 percent in 2004 to 14 percent in 2014. Gun arrests declined from 7 percent to 4 percent over the same period.

The expanding share of immigration arrests highlights the burden of policing immigration crime, which has grown heavier over the last decade. The Department of Homeland Security (DHS), which is responsible for any federal crimes involving the illicit crossing of people and contraband across the U.S. border, surpassed the Department of Justice as the government’s top arresting agency in 2007, Pew reported.

Immigration arrests have risen along with the rapid growth of DHS over the past decade. Customs And Border Protection, the agency which oversees the U.S. Border Patrol, had huge increases in staffing during the mid-to-late 2000s, Pew noted. Between 2004 and 2010, the number of Border Patrol agents jumped from 10,819 to 20,558.

The Bureau of Justice Statistics data also show that federal law enforcement of all types of crime has become concentrated on violations by non-citizens along the U.S.-Mexico border. (RELATED: Federal Law Enforcement Resources Are Concentrated On Immigration And Immigrant Crime, Stats Show)

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In 2014, 61 percent of all federal arrests were of non-U.S. citizens, up from 43 percent in 2004. The share of arrests of U.S. citizens, on the other hand, fell to 39 percent from 57 percent a decade ago.

A disproportionate amount of arrests took place in areas near the southern border. Just five federal judicial districts bordering Mexico–one each in Arizona, California, and New Mexico, plus two in Texas–accounted for 61 percent of all federal arrests , regardless of crime category. Those districts produced 40 percent of federal arrests in 2004.

The Pew report noted that arrest figures for immigration offenses, which include crimes like crossing the border illegally and human smuggling, are not the same as migrant apprehensions or deportations.

Apprehensions are cases in which foreign nationals are caught in the U.S. without proper documentation, and can include both civil and criminal violations. Arrests are cases in which individuals are booked for criminal violations of federal immigration laws, Pew said.

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