Al-Jazeera Video: Gaddafi's invisible force

"Recently, there have been numerous assassination attempts on leading rebels in their eastern stronghold of Benghazi.

Libyans fighting to depose Muammar Gaddafi say he is using hitmen - an invisible force - to carry out targeted killings.

Some documents have been found, detailing those set to be killed. And many more so-called 'Gadaffi lists' are thought to exist.

Al Jazeera's Tony Birtley reports from Benghazi. "