TRIPOLI, Libya—For more than five months in a city locked down by forces loyal to Col. Moammar Gadhafi, regime opponents in Tripoli's Fashloom neighborhood relied on a fellow resistance leader who told them with uncanny accuracy how to evade security sweeps and tipped them off to impending raids against them.

On Thursday, as a rebel advance broke Col. Gadhafi's grip over his capital, the man identified himself to those beyond his underground cell: He is Mahmoud Ben Jumaa, a senior officer in Col. Gadhafi's personal security...