The deaths of at least 53 migrants in Texas, were believed to be the deadliest human-trafficking incident on the southern border in modern U.S. history, and a stark reminder of the human cost of U.S. immigration policies.

The stacks of bodies were found inside or near a tractor-trailer on the outskirts of the city of San Antonio, which is 250 km from the U.S.-Mexico border, a major transit route for people smugglers. The truck deaths cast a spotlight on the usually perilous and sometimes fatal journey north crossing into the U.S. from Mexico. Smugglers often transport large numbers of people in trailers, vans or SUVs through remote areas in sweltering weather.

Local officials believe the trailer was carrying at least 64 migrants from countries such as Mexico, Honduras, Guatemala and El Salvador. At least 40 men and 13 women died and 11 were hospitalized with heat-related conditions. The trailer had traveled through temperatures approaching 40 degrees Celsius.

There has been a growing number of migrant deaths according to the UN's International Organization for Migration. More than 700 migrants crossing the U.S.-Mexico border were dead or missing last year. Apart from unknown causes of death, drowning and harsh environmental conditions are the two leading factors.

Speaking at his daily briefing, Mexican President Andres Manuel Lopez Obrador said, "It happens because there is trafficking of people and a lack of control, in this case at the Mexican-U.S. border, but also in the U.S. interior."

Texas Senator Ted Cruz and Governor Greg Abbott quickly blamed U.S. President Joe Biden for the deaths in San Antonio, saying "These deaths are on Biden. They are a result of his deadly open border policies. They show the deadly consequences of his refusal to enforce the law."

Read more:

Republicans blame Texas migrant deaths on Biden's border policies

One day following the tragedy, Biden called the deaths "horrifying and heartbreaking."

Biden entered the White House in January 2021, promising to tackle immigration challenges that have remained unsolved for decades. Yet since he took office last year, the U.S. has seen a record influx of migrants at its southern border.

There's also been a sharp increase in the number of unaccompanied children crossing the border.

During the COVID-19 pandemic, Biden had kept a Trump-era emergency policy to automatically expel almost all undocumented migrants.

Under the Trump administration, 80 percent or more of monthly migrant encounters, which combine apprehensions and expulsions, at the U.S.-Mexico border resulted in expulsion. But that percentage declined under Biden. In April and May 2022, the most recent two months with available data, more than half of all migrant encounters at the southwestern border ended in apprehension.

In the fiscal year 2021, the U.S. detained more than 1.7 million migrants at its southern border, including 45,000 children.

Read more:

Gruesome truck deaths highlight failure of U.S. border policy

U.S.: From nation of immigrants to graveyard of migrants' dreams