The flood of illegal immigrants crashing the southwest border plunged 23.7 percent in the fiscal year that ended in September, the lowest number in five years, officials said Tuesday.

Experts closely track the number of illegal immigrants caught near the boundary and at entry points along the border for signs of how many foreigners are trying to get into the country. They estimate that for every person apprehended, one slips through into the U.S. interior.

Cracking down on illegal immigration may have been President Donald Trump’s most high-profile promise during the 2016 campaign, and his administration took immediate steps toward fulfilling it. Trump signed a pair of sweeping executive orders, reversing restrictions imposed by his predecessor on both the U.S. Border Patrol and Immigration and Customs Enforcement (ICE) officers.

Perhaps more important, according to immigration experts, the president’s tough rhetoric sent a message to would-be migrants in Mexico and Central America that entering the U.S. without authorization would no longer be tolerated. And they responded. After posting a five-year high in border apprehensions in January — Barack Obama’s last month as president — border crossings dropped off a cliff.

Apprehensions fell to 15,771 in April, the lowest number since the creation of the U.S. Customs and Border Protection agency in 2003.

After that, however, the numbers began creeping up and rose every month through September. Immigration hawks have attributed that partially to the uptick in activity that typically occurs during the summer months. But they agree part of it is the result of a failure by Congress to back Trump’s rhetoric with new tools to prevent illegal immigration — more Border Patrol and ICE agents, more detention space to hold them, and the wall that the president has repeatedly promised to build.

[lz_table title=”Illegal Border Crossings Plummet” source=”Department of Homeland Security”]Apprehensions along southwest border
|Fiscal Year,Total,Change
2017,415.2K,-23.7%
2016,562.2K,26.3%
2015,445.2K,-21.8%
2014,569.2K,16.3%
2013,489.6K,15%
2012,425.9K,30%
[/lz_table]

“We remain concerned about the steady increase in the flow of unaccompanied children and family units from Central America,” Customs and Border Protection acting Deputy Commissioner Ron Vitello told reporters at a news conference. “This trend is enabled by legal and policy loopholes, which are exploited by transnational criminal organizations engaged in human smuggling and trafficking.”

Vitello said that despite a decrease in border crossings, assaults on border officers jumped 44 percent in fiscal year 2017, to 847. He added that agents also seized 2,300 guns, 305,000 rounds of ammunition, and 2.14 million pounds of drugs.

Related: Illegal Border Crossings Rise for Fourth Straight Month

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Smuggling of fentanyl has particularly increased. Vitello said agents seized 1,500 pounds of the highly potent drug in the last fiscal year, up from just 2 pounds in fiscal year 2013.

Vitello said his agency needs the 5,000 additional officers Trump has called for. He also said the agency has test-piloted remote-controlled drones in the Tucson, Arizona, sector for use in dangerous or hard-to-reach areas of the border. He said he hopes that and other measures will supplement the border wall the president wants to build.

“We know, however, that the wall is part of a larger enforcement security system strategy, so we are committed to using new technologies to augment our resources,” he said.