Over all, the data shows that Border Patrol agents apprehended 50,975 people last month, the beginning of the 2019 fiscal year. Customs and Border Protection quietly released the new data on its website on Friday evening without comment.

The new data also follows Mr. Trump’s ordered deployment of 5,600 active-duty military troops to the southwest border. They are there to help stop what had been estimated as 6,000 migrants heading to the United States after fleeing the grinding poverty and violence of El Salvador, Guatemala and Honduras.

Many of the migrants, currently traveling through Mexico, have said they intend to seek asylum in the United States. The number of people in the caravan has declined in recent days, and it is unclear how many will reach the border. The group is still weeks away.

The increase in migrants is certain to put more pressure on Kirstjen Nielsen, the homeland security secretary whose job security is seen as increasingly vulnerable. Mr. Trump has repeatedly complained that Ms. Nielsen has failed to stem the flow of illegal migration that rose after the administration’s first year.

It was unknown if Mr. Trump, who was traveling to Paris, had been briefed on the new apprehension numbers. The president has made clear that he is frustrated by the rising border numbers.