Nearly 52,000 people were apprehended at the southwestern border in May, new data from Customs and Border Protection show, marking the third month in the row that more than 50,000 people have attempted to enter America from Mexico illegally.

The May 2018 count of 51,912 is more than 1.6 times higher than the number of would-be border crossers counted by CBP in May of 2017. Rates of illegal immigration plummeted throughout 2017, an effect many attributed at the time to the election of President Donald Trump, an immigration skeptic. However, the "Trump effect" seems to have abated in 2018, as numbers of illegal border crossers return to 2016 and 2015 levels.

The numbers come despite the Trump Administration having attempted a crackdown on immigration over the past two months. Attorney General Jeff Sessions announced a "zero-tolerance policy" in early April, requiring U.S. Attorneys in border districts to criminally prosecute all eligible illegal crossers. The zero-tolerance policy represents a return to the practice of the late Bush administration and early Obama administration; the Obama administration cut prosecutions in 2014.

Prosecutions along the border have indeed increased since Sessions announcement, according to one recent report. April prosecutions were 30 percent higher compared to March, and 60 percent higher compared to January. However, only about 22 percent of individuals apprehended by the Border Patrol are actually prosecuted, according to the report.

At the same time, the zero-tolerance policy has proved controversial. Due to a preexisting court ruling, federal law enforcement is obliged to not detain children indefinitely while their parents are being held for prosecution. In practice, this has meant that the new policy requires the separation of parents and children, expanding the population of unaccompanied minors for whom the Department of Health and Human Services takes responsibility.

Some 14,000 families attempted to cross the border in May, according to the new estimates from Customs and Border Protection. Any children separated from those families will become the responsibility of the federal government, and will add to the 7,235 unaccompanied minors separately apprehended in May.

The movement of unaccompanied minors across the border represents a major concern for the Department of Homeland Security (which oversees Customs and Border Protection), as they may be being trafficked, and are regardless generally released into the interior on recognizance.