The number of people taken into custody after illegally crossing from Mexico into the United States dropped in June for the first time in six months, plummeting by more than 37,500 since May, according to two officials with firsthand knowledge of the data.

Exactly 94,487 people were arrested crossing between ports of entry in Texas, New Mexico, Arizona, and California last month. The massive decline comes after 132,000 illegal entrants were arrested in May on the southern border, the highest number in one month since March 2006.

Another 650 people were arrested along the U.S.-Canada border and coasts in June. Apprehensions are considered an indication of the number of people attempting to illegally enter the country.

One senior homeland security official who confirmed the numbers said the border is still in a "crisis" because an average of 3,166 people were taken into custody each day in June, but Border Patrol can only hold 4,000 at a time.

Migrants are supposed to remain in custody 72 hours before being transferred to another agency or released into the U.S. But some adults were held by Border Patrol in June "two to four weeks" because they can't be transferred to Immigration and Customs Enforcement facilities due to lack of space, according to a third official who commented separately on the detentions.

The Department of Homeland Security’s Customs and Border Protection arm has not officially released the June numbers. Acting DHS Secretary Kevin McAleenan touted an approximate 25% decline during a press conference in late June. The numbers show a drop of more than 28%.

A federal law enforcement official said McAleenan would be wrong to credit himself or the White House with the decline.

"The reality is that seasonal variations in migration patterns are predictable, and expropriating credit is all too telling of a department desperate to reset after compromising its own mission. There’s been months of foot dragging and real progress on illegal immigration will be made from outside DHS headquarters," the official said in a text.

The DHS did not respond to a request for comment.

McAleenan also said last month the expected decline would be largely due to Mexico stepping up enforcement operations following President Trump’s threat to impose tariffs. However, historical CBP data suggest another reason is also a factor: hot weather. Apprehensions have historically shot up every spring and dropped in the summer due to the brutal heat along the border, as well as health factors among migrants.

In 16 of the past 17 years, CBP documented rises in the springtime months and drops in the summertime, just as took place this year.

Still, despite the decline, the number of people caught illegally crossing the border is well above the 50,000 apprehended in December.

Border apprehensions were around 13,000 per month during the spring of 2017, the year Trump became president.