NEW YORK – U.S. Border Patrol agents made 51,912 arrests in May for illegal crossings, the third month in a row the number has topped 50,000 amid a new “zero tolerance” policy and agents separating families caught crossing illegally.

The Trump administration – like previous administrations – uses the arrest numbers as the best gauge of whether illegal crossings are going up or down, though there is no precise measure of illegal crossings because some people aren’t caught.

The border arrest figures are made up of people who are stopped at land crossings and other official points of entry, according to federal data. The May tally was more than triple the number from April 2017, which was the lowest on record since the Homeland Security Department was created in 2003.

Curbing illegal immigration is an administration priority; Trump cited the issue as an achievement in a tweet earlier this week on his 500th day in office. But behind closed doors last month, he criticized the Homeland Security secretary at a Cabinet meeting for failing to stop rising numbers.

A spokesman for the Department of Homeland Security, Tyler Houlton, said in a statement Wednesday that reversing years of political inaction takes time.

“These numbers show that while the Trump administration is restoring the rule of law, it will take a sustained effort and continuous commitment of resources over many months to disrupt cartels, smugglers, and nefarious actors,” he said.

The debate over immigration reached a fever pitch in recent months following reports that since October hundreds of children crossing the U.S.-Mexico border have been separated from their parents.

The number of separated minors was expected to jump after Attorney General Jeff Sessions said he would enforce criminal charges against people crossing the border illegally with few or no previous offenses. Under U.S. protocol, if parents are jailed, their children would be separated from them.

The U.N. human rights office on Tuesday called on the Trump administration to halt the policy, insisting there is “nothing normal about detaining children.”