The number of people arrested for illegally crossing the U.S.-Mexico border declined by approximately 20,000 in July, the second monthly decline in a row following a steep increase this spring, according to three senior Homeland Security officials and a nongovernmental official.

Apprehensions, or those arrested by U.S. Border Patrol agents for entering the country between official crossing points on the southwest border, dropped more than 20% from June's 95,000 arrests to about 72,000 in July, officials said.

Illegal crossing arrests along the southwest border have dropped about 45% from a peak of 132,000 in May, which was the highest total since 2006. Arrests are an indicator of the rate at which people are attempting to cross illegally.

The Department of Homeland Security is expected to announce the July numbers later this week, likely Thursday.

Officials cited Mexico's stepped up enforcement efforts for preventing a portion of people from passing through the country on the way to the United States. In addition, summer apprehension rates have historically dropped 16 of the past 17 years due to the extreme heat in that region, which prompts many to make the journey during the spring and fall.

Illegal crossings hit a 17-year low in March 2017 when only 12,500 people were apprehended. That number began climbing in mid-2017 from 20,000 to 30,000 arrests each month. It surpassed an average of 40,000 arrests per month by the spring of 2018 and continued climbing to around 50,000 each month last fall. In February, arrests spiked to 66,000 before dramatically jumping to 92,000 in March and 99,000 in April before reaching May's 13-year high.

Acting Homeland Secretary Kevin McAleenan was picked by Trump in January 2017 to serve as the country's top border official and was confirmed by the Senate in March 2018. He served at Customs and Border Protection from early 2017 and was moved to Homeland Security in April.

Border arrests do not include those who tried to pass through a port of entry but were denied admittance. Since the start of the fiscal year, the number of people turned away at ports has remained consistent at between 9,000 and 11,500 due to CBP's metering the number allowed to present themselves for consideration.