Story highlights The drop in March was a sharp departure from annual trends

The numbers marked a 35% drop from February

Washington (CNN) Apprehensions at the southern border continue to fall dramatically, according to new numbers from Customs and Border Protection -- a drop that experts attribute to President Donald Trump's aggressive immigration enforcement policies.

There were roughly 12,000 total apprehensions at the southwest border in March, according to numbers obtained by CNN that are expected to be released this week. That represents a 35% drop from February and a 63% drop from March 2016.

In 17 years of CBP data , apprehensions had never dropped from February to March, typically rising slightly.

Former Acting Customs and Border Patrol Commissioner David Aguilar said Tuesday at a hearing in front of the Senate's Homeland Security Committee that through the end of March, immigration officials have seen a 67% drop in illegal crossings from Mexico as compared to the same period last year.

Aguilar attributed the drop to Trump's hardline position on immigration -- a focal point of his campaign and the first few weeks of his presidency.

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