(CNN)Thirteen people have been charged as material witnesses in the case against James Matthew Bradley Jr., who is being prosecuted in the deaths of 10 undocumented immigrants in an overheated semi-truck in San Antonio, the US Attorney's Office of the Western District of Texas said Wednesday.
13 held as material witnesses in Texas trailer deaths
All 13 remain in federal custody, but prosecutors have not released their identities. They are being represented by San Antonio-based attorney Michael McCrum, the US attorney's office said.
Material witness laws allow officials to detain those who may have knowledge of crimes committed by others to ensure they will testify in legal proceedings, including grand jury sessions and trials.
The tractor-trailer was found parked at a Walmart in San Antonio early Sunday morning after an employee at the store called police.
Authorities found eight people already dead and another 30-40 injured, many of whom had suffered heat stroke or other heat-related injuries, officials said. Two others died at the hospital shortly afterward.
The driver of the tractor-trailer, Bradley, said he was not aware of the cargo in his vehicle and only discovered it when he parked at a Walmart and went outside to urinate.
The 60-year-old was charged on Monday with knowingly transporting undocumented immigrants.
Bradley's detention hearing, which had been scheduled for Thursday, has been canceled.