Migrants from Guatemala are seen on the banks of the Rio Bravo river before crossing illegally into the United States to turn themselves in to request asylum in El Paso, Texas, June 11, 2019.

A top Trump immigration official on Tuesday said nearly 1 million migrants were taken into custody along the U.S. southern border in fiscal year 2019.

The figure marks the highest volume of arrests there since 2007 due to an influx of Central American families, Mark Morgan, acting commissioner of U.S. Customs and Border Protection, said during a White House briefing. The apprehensions peaked in May.

Morgan added that about 52,000 migrants were taken into custody in September, marking the fourth consecutive month of declines in the number of migrant arrests. September's figure is more than 60% lower than the peak of 133,000 arrests in May.

Just over 500,000 were apprehended along the southern border in fiscal year 2018, according to government figures.

President Donald Trump has made reducing illegal immigration a top priority of his administration and reelection campaign. The latest figures indicate his immigration crackdown may be helping to reduce the number of migrants apprehended at the southern border.

Morgan also added that the administration has essentially ended the "catch and release" of migrants by sending them back to Mexico through a program known as Migration Protection Protocols. Nearly 50,000 migrants have been sent back to Mexico under MPP, which directs migrants to dangerous border cities to await their individual immigration trials.

"We are closing those loopholes," Morgan said. "If you are coming here as an economic migrant, you're not going to be allowed in the United States, and that's driving a lot of people in return."

Presidential candidate Julian Castro slammed MPP and tweeted Monday that Trump's agenda is "killing people - and it's on purpose."

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As to whether the United States is moving forward with a potential safe third country deal with Mexico, Morgan said the cooperation from Mexico is "exactly where we need it to be."

He added that it's ultimately Congress's responsibility to pass legislation to address the southwest border crisis.

"At the end of the day, Congress's failure to act, is the issue," Morgan said. "To have a durable, meaningful solution with respect to this crisis, Congress has got to get off the bench, work on a bipartisan matter to pass meaningful legislation to address this crisis. I've said that and I'm going to keep saying that."