The abandoned truck that carried migrants across the southern border in a botched smuggling job that left 53 people dead had successfully passed through a U.S. Border Patrol checkpoint inside America, an anonymous official told the Associated Press. The sweltering tractor-trailer had gone through a checkpoint on Interstate 35, around 26 miles inland from the border city of Laredo. The smugglers are believed to have surveilled the checkpoint and timed their passage to take advantage of a shift change, when traffic was backed up, a source close to the investigation told the San Antonio Express-News. Citing the anonymous official, AP also reported there were a total of 73 people in the trailer when authorities arrived at the scene of unimaginable horror in San Antonio on Monday. It wasn’t clear if the truck had been stopped at the U.S. checkpoint or allowed to drive on without a thorough inspection. Homero Zamorano Jr., 45, appeared in court Thursday accused of being the driver—he will potentially face the death penalty if convicted of his smuggling charge.