MEXICO CITY — Mexican prosecutors on Wednesday finally secured convictions and 50-year prison sentences against 11 drug cartel gunmen accused of carrying out a massacre of 122 passengers in 2010 and 2011, who were dragged from passing buses and forced to fight to the death with sledgehammers.

The verdicts announced on Wednesday concern one of the most gruesome chapters of Mexico’s drug war. They were so horrific that it was hard to believe them until dozens of bodies with their skulls smashed in were found in unmarked graves.

Federal prosecutors said the 11 suspects were arrested between 2015 and 2017 and have been in prison since then, but their trials lasted between seven and nine years, which is not too unusual in Mexico.

Prosecutors in Tamaulipas state said at the time that members of the now-fragmented Zetas cartel had begun dragging male passengers off buses bound for the border city of Reynosa, which lies across the border from McAllen, Texas, or Matamoros further east.

Officials said at the time that the Zetas suspected the rival Gulf Cartel was sending reinforcements by bus to border towns they controlled. The Zetas took young men off the buses, interrogated them and offered some the chance to survive and join the gang – if they proved their worth by fighting other innocent passengers with sledgehammers.

The extent of this cruelty seemed unimaginable until forensic scientists began to uncover scattered mass graves containing hundreds of bodies. Almost all of them were young men, many with their skulls smashed in. A number of hammers were also found in the graves.

This tragedy first came to light in 2011, when authorities in the northern border state of Tamaulipas discovered 48 clandestine graves containing the bodies of 193 people, most of whom had had their skulls smashed in with sledgehammers, and many of them were migrants from Central America.

It was later revealed that the victims had been dragged from passing buses by the old Zetas drug cartel and forced to fight each other with hammers, or they would have been killed if they refused to work for the cartel.

The control of the drug cartels in Tamaulipas was so comprehensive at the time that the bus companies threatened by the gangs only reported the missing people when the victims’ unclaimed luggage began to pile up at the bus stations in the border towns.

The kidnappings and murders took place in and around the Tamaulipas city of San Fernando, where the Zetas also slaughtered 72 migrants, many of them from Central America, around the same time. The sole survivor later told authorities that cartel gunmen killed the migrants after they refused to join the gang.