(CNN) Arrests of migrants illegally crossing the US-Mexico border dropped in December, the seventh month of decline in a row.

In December, US Border Patrol apprehended around 32,800 people on the southwest border, according to preliminary Border Patrol data obtained by CNN, down from 33,510 in November.

In mid-December, acting Customs and Border Protection Commissioner Mark Morgan , who oversees Border Patrol, warned that border apprehensions were "still at crisis levels." He said that the average apprehensions were a little over 1,400 daily at the time. Apprehensions are used as a measure of illegal crossings and overall migration trends.

CBP does not comment on monthly numbers until they are officially released by the agency.

Apprehensions increased during the first half of the month of December, but there was a big drop after December 15, according to a Border Patrol source familiar with the numbers.

Read More