By Tom Gibb BBC correspondent in Sao Paulo

Violence in Brazil involving drugs gangs has flared before

The judge overseeing the prison where some of Brazil's top criminal leaders are being held has been ambushed and murdered.

The killing came just a few hours after the government announced measures to strengthen the federal police in an effort to combat the growing power of organised crime.

The authorities believe the judge may have been killed by Rio de Janeiro's largest drugs cartel which last month unleashed a wave of violence in the city.

Judge Jose Antonio Machado Dias was gunned down as he drove through early evening traffic in the city of Presidente Prudente in Sao Paulo state.

He was the local judge whose responsibilities included transfers and benefits at Brazil's newest high security jail in the city.

Organised crime's power

Last month, Fernandinho Beira-Mar, the imprisoned leader of Rio de Janeiro's largest drugs cartel, the Red Command, was moved to the jail after Rio's authorities accused him of orchestrating a wave of violence in the run up to carnival.

The authorities now fear that the Red Command - or one of its allies - murdered the judge.

If so, this shows the growing power of organised crime in Brazil.

One of the problems is that police and prison policy are the remit of local state governments.

In many places security experts say corruption and political rivalries prevent effective action.

The government now says it intends to beef up Brazil's federal police, recruiting 5,000 new agents and turning them into the equivalent of the Federal Bureau of Investigation in the United States.