MEXICO CITY – Armed men hijacked a bus carrying 34 detained Cuban migrants in southern Mexico after forcing immigration agents away at gunpoint, officials said on Thursday.

Eight assailants blocked a road in southern Chiapas state late Wednesday, stopping the bus carrying the Cubans and forcing the unarmed immigration agents to get off, said an official of Mexico's National Immigration Institute. None of the agents were harmed.

The official, who was not authorized to be quoted by name, said the assailants “were carrying heavy caliber weapons, and threatened the agents” with the guns.

The bus was later found empty near the jungle city of Ocosingo.

The Cuban migrants had been detained on June 5 on a boat off Mexico's Caribbean coast near Cancun.

The official said he did not know who carried out the attack or why. He added that he didn't know whether the assailants were immigrant smugglers seeking to recover their charges. Migrant traffickers have also been known to kidnap migrants from their rivals and later ransom them off.

Cubans are increasingly traveling through Mexico to try to reach the U.S. instead of trying to get past Coast Guard patrols off Florida.

The seized migrants were being taken to an immigration processing center in the nearby city of Tapachula when the hijacking took place. Mexico's immigration agents are normally unarmed on such assignments.

Some undocumented Cuban migrants who reach Mexico's coast, usually in boats or makeshift rafts, are allowed to remain in the country, where they quickly make their way to the U.S. border.

However, some are returned to the island.