Mr. Canseco said it was unclear why gangs were removing people from buses. They may have been trying to forcibly recruit passengers as foot soldiers, or they may have intended to hold them for ransom, he said. It may also be that the missing men were heading to the United States and that the gangs took them off the bus to extort payment to get them across the border.

Security forces rescued five people being held by a gang, all whom had been kidnapped from a bus, Mr. Canseco said. They helped lead the security forces to the mass graves.

The bodies have not yet been identified, and so it has not been confirmed that they were those of other kidnapped bus passengers. Soldiers have arrested 14 people in connection with the mass graves, said Alejandro Poiré, the security spokesman for the federal government. “There are lines of investigation and information generated from the arrests to lead to the full identification of the entire criminal cell responsible for these acts,” he said.

The federal government will reinforce security along the highway, he said.The graves were found in the municipality of San Fernando, where 72 migrants from Central and South America were killed last August.