MEXICO CITY (AP) — Mexican immigration officials said Thursday they have found two migrants dead in the desert near the Texas border, and are looking for two more migrants that survivors said had also died.

All of the migrants were Mexican citizens. The deaths occurred in the sparsely-populated northern border state of Coahuila, across from Texas.

The National Immigration Institute said it had received a report from U.S. Customs and Border Protection on Tuesday saying that 12 migrants had been detained at the border.

The detained migrants said that they were in a group of 16 that had been abandoned in the desert by migrant smugglers. The twelve said the other four had died, though the cause of death was unclear.

Mexican authorities launched an air and land search for the migrants, and said they considered the other two “missing.”

Because it is practically impossible to carry enough water to make it through the desert, deaths from dehydration and heatstroke are not infrequent. However, as described by immigration officials, the area where the bodies were found is less traveled and well outside the main smuggling routes.