Correspondent Quill Lawrence reports on the killing

Prison officials said the gunman approached Helmer "Pacho" Herrera as he was playing football in the exercise yard, hugged him, and then shot him in the head.

Fellow prisoners jumped on the man and beat him unconscious.

Herrera was one of the top five leaders of the Cali gang, which once controlled some 80% of the world's cocaine trade.

Correspondents say the motive for his murder was not immediately clear.

Crackdown

Herrera was the cartel's "Number Three"

Helmer Herrera turned himself in to police on 1 September 1996, during Colombia's crackdown on members of the Cali drugs cartel.

His seven year sentence was doubled last October in light of new evidence.

The United States had requested the extradition of Herrera, but Colombia's extradition law only affects crimes committed after the law passed, in December of 1997.

American authorities accuse Herrera of having exported more than 3,000 kilos of cocaine to the United States.

The government of President Ernesto Samper put all of the chiefs of the Cali drugs cartel behind bars.