By Alicia A. Caldwell , Michelle Hackman , Alyssa Lukpat and Talal Ansari

'Federal officials called the episode an ‘alleged human smuggling event.'

SAN ANTONIO—Three people are in custody following the death of 51 migrants who were among those found in a sweltering tractor trailer here Monday, officials said.

One of those arrested was the driver of the tractor trailer, according to U.S. Rep, Henry Cuellar, who said he had been briefed by Homeland Security officials. Mr. Cuellar, whose district is nearby, said about 100 migrants were packed into the 18-wheel vehicle in the South Texas border city of Laredo after they crossed the border illegally. They then passed a Border Patrol checkpoint before reaching San Antonio.