The guards, among 18 on duty at the time, waited 2 hours before reporting the brutal riot so no external authority was aware of what was happening until the prisoners -- members of the country's deadliest criminal gang, the Zetas -- were far away from the maximum-security Apodaca prison, just north of the major industrial city of Monterrey, Nuevo Leon state public security spokesman Jorge Domene Zambrano told reporters.
The 18 guards, along with four top prison officials, have all been fired and are under criminal investigation for alleged complicity in the riot and jailbreak, Domene said.
All the inmates killed were from the Zetas' bitter rival, the Gulf Cartel, Domene said.
Two of the escaped inmates were identified as Oscar Manuel Bernal, alias "the Spider," the Zetas drug lord in Monterrey, and Chacha Rogelio Quintanilla, known as "the Yeyo," the Zetas drug lord in Guadalupe, east of Monterrey, officials said.
The Zetas and the Gulf Cartel, former allies, have been warring for two years for control of drug routes and lucrative drug markets, especially in the northeastern Mexican states of Nuevo Leon, where Monterrey is located, and Tamaulipas, which borders Texas.
The Zetas were once Gulf Cartel enforcers.
Authorities also revised the record to show the riot started shortly after 1 a.m. CST Sunday, rather than around 2 a.m., and said some of the dead were killed with "accurate shots to the head," the Mexican newspaper El Universal reported.
Officials earlier said no firearms were found among the prisoners. They described the dead Gulf Cartel inmates as having been attacked with sharp instruments, stones and clubs.
As the violence spread from one cellblock to another, the 30 Zetas escaped, allegedly with inside help from guards or other officials, authorities said.
"This isn't a thing where, in the middle of a riot, it occurred to these people to escape," Nuevo Leon Gov. Rodrigo Medina said at a news conference Monday.
"Without a doubt there was premeditation," Medina said. "This was planned."
He said it was hard to imagine "the treason, corruption and complicity of a handful of officials that obstructs the good work of police, soldiers and marines who risk their lives every day."
As police and soldiers combed the state searching for the escaped prisoners, Medina said Nuevo Leon was offering an $800,000 reward for information leading to the fugitives' capture.
Apodaca prison has a capacity to house 1,700 inmates but was holding about 2,700 at the time of the riot, Domene told The Wall Street Journal Sunday.
Medina asked Mexico's federal government to help relieve Nuevo Leon's prison stress by taking as many prisoners as possible to federal institutions.
The riot occurred four days after a fire at an overcrowded penitentiary in Comayagua, Honduras, killed at least 382 people, making it the world's deadliest prison fire. The second-most-deadly was at the now-closed Ohio Penitentiary in Columbus, April 21, 1930, which killed 322.
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