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HEALDSBURG, Calif. — The number of people crossing illegally into the United States from Mexico has dropped by 50% in the past month, authorities said Tuesday, as President Joe Biden comes under growing pressure from both parties over security at the border.

U.S. Customs and Border Protection said it had encountered migrants between ports of entry 124,220 times in January, down from more than 249,000 the previous month.

The figures do not change the fact that the number of people crossing into the United States has reached record levels during the Biden administration, and crossings typically dip in January. Immigration trends are affected by weather patterns and other issues, making it difficult to draw conclusions from monthly numbers.

But the drop in crossings was a glimmer of good news for the Biden administration as House Republicans impeached Alejandro Mayorkas, the homeland security secretary, Tuesday on charges of willfully refusing to enforce border laws.

The figures also amounted to a respite for some large American cities grappling with the burden of sheltering migrants.

Troy Miller, acting head of the border agency, said the drop in border crossings is the result of “seasonal trends, as well as enhanced enforcement efforts” by Border Patrol and “our international partners.”

In late December, Biden sent Secretary of State Antony Blinken and other top U.S. officials to Mexico City, where they met with President Andrés Manuel López Obrador to find a way to slow the surge in illegal crossings.

Immigration has taken on enormous political importance as this year’s presidential election approaches. Biden has blamed his predecessor and putative challenger, former President Donald Trump, for undermining a bipartisan immigration deal in Congress that would crack down at the border.

Immigration experts say they expect another rise in numbers soon. Casa Alitas, a Catholic agency that runs several shelters in Tucson, Arizona, said numbers have been climbing again.

In October, November and December, the shelter network was receiving about 1,000 migrants each day. That number plummeted to an average of about 500 daily in the first three weeks of January. This week, numbers were in the 1,000 range again.

This article originally appeared in The New York Times.