According to the local fire department, 46 suspected migrants were found dead in a truck in the US state of Texas.
16 survivors were taken to hospitals, San Antonio Fire Chief Charles Hood said Monday night.
US media initially reported more than 20 dead and then more than 40 dead.
Television pictures showed a large-scale operation by the police and rescue workers.
Migrants are repeatedly smuggled from Mexico to the United States in trucks.
The arch-conservative governor of Texas, Greg Abbott, spoke in the short message service Twitter, referring to a media report of 42 deaths and blamed President Joe Biden's migration policy for it.
"These deaths are Biden's fault".
They are "the result of his deadly open borders policy," Abbott said.
It has "deadly consequences" if the law is not enforced.
Controversial migration policy
The opposition Republicans accuse Biden of taking too lax a course in migration policy and in securing the southern border.
They also want to use this issue in their campaign for the midterm congressional elections in November.