(CNN) When 10 smuggled migrants died in San Antonio from a scorching hot semi truck with almost no air to breathe, the country was appalled.

But their deaths are just a fraction of how many people die each year trying to flee their homelands and sneak into the US.

Every year, hundreds of undocumented immigrants perish trying to make the journey. And virtually all of those deaths occur while being smuggled, Customs and Border Protection spokesman Mike Friel said.

Friel said virtually all those crossing the border illegally are led by smugglers. That's because cartels have a stranglehold on the Mexican side of the border and control much of the land, he said.

"There are very few people who attempt to cross on their own," Friel said, and virtually no one would "risk going against the cartel."

Migrants walk along train tracks in Sonora state, Mexico, on January 13, 2017. Hundreds of Central American and Mexican migrants try to cross the US border daily.

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