John Massey is understood to have hid in the prison's gym yesterday until he could sneak on to the roof and escape over the wall without being stopped.
It is thought his makeshift rope was made from bedsheets.
Pentonville was described by prison inspectors as one of Britain's most challenging jails to run last year, but the inspection also found "no obvious problems with physical security".
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."