A top Libyan rebel commander was shot and killed by unknown assailants on Thursday, just hours after he had been detained for questioning by the rebel leadership in Benghazi, threatening to plunge the rebel ranks into chaos and infighting.

Opposition leader Mustafa Abdel Jalil told a news conference late Thursday night that Abdel Fatah Younis and two officers working for him were gunned down by assailants and that the head of those assailants had been arrested, but offered no further details.

Minutes...