The killing of General Francois al-Hajj marks a further dramatic deterioration in Lebanon's internal turmoil, coming as it does at a time when the country was already suffering political impasse over its future president.

Significantly, General al-Hajj is the first Lebanese military figure to be targeted by bombers since 2005 when this current wave of assassinations began with the killing of Rafik Hariri, a former Prime Minister.

General al-Hajj, a high-ranking figure, had been tipped to replace the country's military commander if, as has been widely speculated, General Michel Suleiman takes over as president.

The latest assassination is an important watershed, as - despite the prominent divisions within Lebanese society - the military had previously been regarded as a unit that knitted the country together.

Because security is always an issue in Beirut, the Army is always held up on a pedestal as a unifying force and it is almost taboo to criticise it. The Army lost 160 soldiers in the battle against Fatah al-Islam (an Islamist extremist group that was holed up in a refugee camp) and there was a wave of public gratitude for their sacrifices afterwards.

Attention will now turn to who committed this bombing and there are a number of possible candidates.

On the one hand there are cells of Fatah al-Islam, which Mr al-Hajj defeated at the Nahr al-Bared camp, still dotted around other Palestinian refugee camps in Lebanon. The United Nations forces positioned along the Israeli border are known to be very concerned about this. It would not be a surprise if this radical group claimed responsibility for this bombing in the coming days, regardless of whether or not it actually committed the bombing.

Another possibility would be pro-Syrian militants within Lebanon, who are believed to have been behind the killings of a number of anti-Syrian politicians in the past two years. Such groups are unlikely to be a direct part of the mainstream pro-Syrian opposition Hezbollah group, which says it wants to oppose the Government by democratic means, but may well be more shadowy characters from the extensive and murky world of Lebanese militancy.

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