As part of the BBC News website's series about prisons in Latin America, Mariusa Reyes reports on the situation in Mexican jails, where criminals are said to remain active despite being behind bars.
It is morning at the North Preventive Jail, one of Mexico City's 11 jails. In the admissions office, guards are busy signing in many new inmates and checking on their belongings, which they call "remittances".
The North Preventive Jail is overcrowded, as are the rest of Mexican prisons. It holds 9,300 people - but it was designed to hold half that number.
Jails in Mexico are overcrowded
But overpopulation is not the main problem here - it is crime.
This is a medium-to-high security prison. Inmates here have been charged with murder, sexual abuse, kidnapping, robbery and drug-trafficking. Many have yet to be sentenced.
There are only four guards to watch over 300 prisoners, all of whom are considered very dangerous.
"It is a tense situation, inmates are usually aggressive," says Martin Cordoba Gomez, supervisor of the jail's maximum security area.
Mr Cordoba, who has been working at the prison for more than a decade, adds that: "Guards need to learn how to approach them."
Guns, drugs, money and mobile phones are frequently smuggled into Mexican jails.
The authorities acknowledge that crimes such as kidnapping, drug-smuggling, as well as attacks on rival gangs in Mexico City and other parts of the country, are sometimes planned from within prisons walls.
They believe, for example, that the abduction in July of the Cruz Azul football team's manager, the Argentine Ruben Omar Romano, was masterminded in a jail.
"Members of big or small gangs manage to remain active despite being behind bars. They usually operate via their visitors or lawyers," says Armando Aguirre, security chief at the North Preventive Jail.
The authorities admit that "vulnerability" and "permeability" are significant problems in Mexico's penitentiary system.
"It is pretty evident that some inmates keep on committing offences despite being in prison, possibly with the help of relatives or friends who visit them," says the jail's director, Armando Mendez Gutierrez.
Prisoners also communicate with the outside world using mobile phones that have been smuggled in and the 70 public telephones available at the detention centre, according to the authorities.
Mexico's police force is currently testing technology to block mobile signals in the jail and track down calls made from public phones.
But some claim that corruption and complicity of penitentiary officials are also to blame.
"All sorts of crimes are committed in this prison: robbery, murder, prostitution, and drug and alcohol trafficking," an inmate who asks not to be named says.
Inmates communicate with the outside world using mobile phones
"Those who have money can do whatever they want. Many inmates have accomplices outside who follow their orders", he added.
Relatives are allowed to visit four times a week, but some see their loved ones less frequently. This is because they cannot afford expenses like travel, food and other articles they may want to bring to the inmates.
Some claim they have to have extra money for bribes in order to smuggle in prohibited items.
"Anything can be smuggled in," says Sonia, whose nephew is serving a prison term.
"There's corruption everywhere. Things don't work for us if we take the right path."