Six out of 10 Mexican prisons are "self-governed" by prison gangs or drug cartels, according to a report issued Monday by the country's human rights commission.
Commission representative Andres Aguirre said at a news conference that Mexico's prisons also are plagued by overcrowding, a shortage of guards and corrupt employees who sometimes help with breakouts.
Aguirre said 60 percent of the country's 430 prisons or jails are controlled by criminal elements.
He added that the escape of 521 inmates over 14 incidents since 2010 - often with the help of corrupt prison officials - constitutes a grave problem for the country.
Initial reports indicated the Piedras Negras inmates escaped through a 23-foot-long (2.9 meters) underground tunnel, but it was later revealed that they had merely walked out the facility's front door with the help of prison guards.
"A good number of (Mexico's) prisons feature similar conditions to those at Piedras Negras," Aguirre said.
Overcrowding of the country's prisons and jails is another problem facing the national penal system.
As of July, Mexico's overall prison population stood at 48,872 inmates, or 25 percent above the system's official capacity.
Mexico also faces a shortage of prison guards relative to international norms, Aguirre said.
The commission's findings are a reminder of the challenges that await Enrique Pena Nieto, Mexico's incoming president, who has pledged to reduce crime in the country after six years of increased gang-related violence under President Felipe Calderon.
Reporting by Lizbeth Diaz; Writing by David Alire Garcia; Editing by Bill Trott