Carandiru prisoners have frequently rioted

Last year nearly 3,500 inmates escaped from prisons in the state of Sao Paulo, more than were released after completing their sentences.

So far this year 1,300 have escaped and last week 106 got out of Carandiru, the main prison in Sao Paulo, in the biggest ever mass escape through a tunnel.

After crawling through the tunnel under the prison walls, they then walked over two miles through sewers to freedom.

Bribery

An investigation by police and public prosecutors has shown that most escapes are planned and organised by gangs outside prison and cost between $4,000 and $8,000 per prisoner.

Part of this money goes in bribes for prison staff.

Tunnels are one of the most popular methods - this year alone 31 tunnels have been discovered in the Carandiru prison.

Some escapes have been spectacular, involving armed attacks on jails or ambushing prisoner transports on highways.

Some have been simple - walking out dressed as a woman visitor or in the uniform of a guard.

Mobile phones confiscated

Mobile phones smuggled into prisons now play an important part in many of the escapes.

Over 800 of them have been confiscated from prisoners in the last two years.

According to Sao Paulo's director of prisons, those who are escaping are the most dangerous prisoners, the ones who can afford to pay.

In an effort to make escaping more difficult, the prisoners in this category are now being transferred from the capital to a maximum security prison hundreds of kilometres away.