(CNN) Border Patrol arrests on the US-Mexico border continued to drop in September, marking a steady decline since the high earlier this spring when Trump administration officials struggled to stem the flow of migrants attempting to enter the US.
There were nearly 40,000 arrests on the southern border in September, which was the lowest month this fiscal year, according to a source familiar with the data. For comparison, there were nearly 133,000 apprehensions in May.
Apprehensions along the southern border were higher in fiscal year 2019 than in any fiscal year since 2007, with slightly more than 850,000 arrests, according to Border Patrol data.
Border Patrol apprehensions, which are used as a measure of illegal crossings, are particularly important to President Donald Trump, who views them as a barometer of the situation along the southern border, according to administration officials.
There was a dramatic spike in border crossings this year, led by an uptick in families and children from the Northern Triangle countries of Guatemala, Honduras and El Salvador coming to the US. The surge caused severe overcrowding in Border Patrol facilities and led to a number of administration measures aimed at reducing the flow of migrants.
"(B)y the end of the fiscal year, we will see numbers more than triple the record for family units arriving at the border, with over 500,000, and record number of unaccompanied minors," said acting Homeland Security Secretary Kevin McAleenan in prepared remarks on Monday.
The administration's responses to the surge included ramping up its policy of returning migrants to Mexico, threatening Mexico with tariffs, building temporary tent courts along the southern border and diverting billions of dollars in military construction funds to build a border wall.
Officials have credited a combination of policy initiatives for the decline but have repeatedly called for more permanent legislative solutions.
Daily arrivals at the border are down 65% from the peak in May, and total enforcement actions for Central Americans arriving at the border have dropped by more than 70%, said McAleenan.
Historically, apprehension numbers have risen in the spring and dropped in the fall and winter. However, in recent years, there have been severe lows and highs, not fitting into a seasonal pattern.
The Washington Examiner first reported on the monthly apprehension numbers.
Customs and Border Protection, which oversees US Border Patrol, does not comment on unofficial numbers.