EL CENTRO, Calif. — When word spread that more than a dozen people had been killed in a crash at an intersection about two miles northwest of the small town of Holtville, some thought it might have been farmworkers headed to start their long day in the nearby agricultural fields. But things didn’t add up. The vehicle was all wrong. Not a van or a bus. The number of people crammed into the SUV was far too high for a vehicle that normally would hold seven or eight passengers. And then there was the debris field. Missing from it was a tell-tale sign, for some advocates, of farmworkers...