(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.

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