Despite an apparent human smuggling operation that resulted in the gruesome deaths of nine in a tractor-trailer in Texas, the number of people apprehended for illegally crossing into the United States from Mexico remains sharply lower in recent months compared with past years.

In June, 21,659 people were arrested or turned away at U.S. ports of entry along the Mexican border, according to Department of Homeland Security statistics. That's slightly higher than May but less than half the number from June 2016. June marked the fifth consecutive month that the numbers were markedly lower than in 2016.

Early Sunday, eight bodies were found in a truck in the parking lot of a San Antonio Walmart. At least 30 more people in the truck were rushed to hospitals, many in critical condition. One of those died later Sunday, officials said.

The temperatures Saturday had exceeded 100 degrees, and the truck had no functioning air condition, authorities said.

"We’re looking at a human trafficking crime," San Antonio Police Chief William McManus said.

The driver was arrested, and Homeland Security's Immigration and Customs Enforcement (ICE) was leading the investigation.

“By any standard, the horrific crime uncovered last night ranks as a stark reminder of why human smuggling networks must be pursued, caught and punished," ICE Acting Director Thomas Homan said in a statement.

President Trump's highly controversial immigration crackdown has meant an increase in arrests of undocumented immigrants already in the country in recent months, but arrests along the border began their precipitous fall in February. Homeland Security Secretary John Kelly said that decline reflected an overall reduced flow of illegal immigrants across the border.

Kelly credited Trump's executive orders such as those aimed at jump-starting construction of the U.S.-Mexico border wall and expanding the powers of federal immigration officials to arrest undocumented immigrants. Courts have turned aside some of Trump's orders, but the atmosphere alone created by the Trump administration's policies could be curbing immigration enthusiasm.

"Since President Trump took office on Jan. 20, we have seen a dramatic drop in numbers," Kelly said in March. He said the decrease was also encouraging because it meant "many fewer people are putting themselves and their families at risk of exploitation, assault and injury by human traffickers and the physical dangers of the treacherous journey north."

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Kelly said tightened border security has driven up the fees charged by human smugglers. Since November 2016, “coyotes” have more than doubled their prices in some areas: $8,000 from $3,500 in certain mountainous regions.

"The early results show that enforcement matters, deterrence matters, and that comprehensive immigration enforcement can make an impact," he said.

The vast majority of human smuggling cases are handled outside the spotlight. In fiscal year 2016, ICE's Homeland Security Investigations unit initiated 2,110 human smuggling investigations resulting in 1,522 criminal convictions.

In Sunday's incident, police were alerted to the tragedy when someone from the truck asked a Walmart employee for water. The employee called police.

A similar case in Texas in May 2003 proved even more deadly when 19 immigrants died inside a sweltering tractor-trailer. Prosecutors in that case said the driver heard the immigrants screaming for their lives but refused to free them. He was initially sentenced to multiple life terms of life in prison, but an appeals court overturned the sentence, and in 2011 he was re-sentenced to 34 years in prison.

Homan personally worked on that case.

"So long as I lead ICE, there will be an unwavering commitment to use law enforcement assets to put an end to these practices," he said.