Border Patrol arrests fell sharply in June to the lowest level since February, according to the Department of Homeland Security.

The agency attributed the drop to the "administration's zero-tolerance policy."

"US Customs and Border Protection's Southwest Border Migration numbers declined by 18 percent when compared to the previous month," DHS said in a press release.

The agency made 34,057 arrests on the border with Mexico during June – that's down 16 percent from the 40,344 in May, according to the official, who spoke on condition of anonymity because the numbers are not yet intended for public release.

Arrests were still more than double the 16,087 made in June 2017, but the sharp decline from spring could undercut the Trump administration's narrative of a border in crisis.

The June tally is preliminary and subject to change.

"DHS will continue to enforce the rule of law and uphold our nation's immigration laws as passed by Congress," the release states. "As we have said before, the journey north is dangerous and puts individuals in the hands of smugglers and traffickers."

Meanwhile, the agency continues to call on Congress to address the crisis at the border by closing legal loopholes that drive illegal immigration.

Customs and Border Protection, which includes the Border Patrol, declined to comment on the numbers, saying it doesn't discuss them as a matter of policy until public release "to ensure consistency and accuracy."

The Customs and Border Protection commissioner, Kevin McAleenan, told agents to temporarily stop referring illegal entry arrests to the Justice Department for prosecution if they involve parents unless those parents had a criminal history or the child's welfare was in question.

His edict came within hours of the president's directive to avoid splitting families.

Hot summer temperatures also could be a major influence, discouraging people from walking in the scorching and potentially lethal heat in much of Arizona, California, New Mexico and Texas.