I posted an earlier version of this piece on Saturday. However, some readers were confused that the earlier post was combined with some other thoughts on the NSA. So, paying heed to developments from over the weekend, here's an updated new post without the NSA material!
Friday's bombing in Beirut was almost certainly the work of the Syrian GSD and/or Hizballah. I make that argument based on three factors.
1) Hizballah is under extraordinary physical and political pressure inside Lebanon. The group is thus desperate to reassert its domestic power-perception position. Because of Chatah's symbolic representation of the March 14 bloc and his well known association with the Hariris, he offered an opportune target from which Hizballah could broadcast their overarching message - 'we are prepared to wash the streets with blood in order to maintain our power'. Let's be clear, this is political terrorism in its most unambiguous form.
2) The Special Tribunal for Lebanon (established to investigate the Hariri assassination and a number of other attacks) will shortly begin its trial (in absentia) of a number of Hizballah operatives. Those accused include Mustafa Badr al-Din (the suspected head of Hizballah's foreign operations directorate)*. In this regard, Chatah's assassination was likely intended to send another basic message to the international community - Hizballah will not acquiesce to outside pressure.
3) The Lebanese Hizballah has a long history of engaging in assassinations against its political opponents. Further, the Syrian Civil War provides abundant proof that the group has little compunction about massacring anyone who stands in their way (a truth that I once underestimated). As SOP, Hizballah denies any responsibility for these kind of atrocities. Still, their record is transparent.
On a related note, it's critical that we pay heed to Siniora's evident fury (which reflects popular concerns) over what's just happened. In articulating that the March 14 bloc will push for restrictions on illegal arms, Siniora is (as the Daily Star notes) issuing a thinly veiled threat to Hizballah. In short, he's laying the foundations for a tougher counter-response. As a corollary, Saudi Arabia's grant of $3 billion to the Lebanese Army is surely designed to provide a counter-weight to Hizballah. The walls are closing. In further vein, it will be interesting to see how Michel Aoun reacts to this attack. Although he remains largely sympathetic to Hizballah, Aoun has also publicly flirted with the notion of a rapprochement with the Future Movement. Should Aoun decide to move towards Siniora/Hariri, even in a limited way, this assassination could cause major blowback for Hizballah. After all, it's hard to underestimate how much Hizballah relies upon Aoun; both for political support and cross-sectarian political cover.
*Interestingly, much of the evidence against Badr al-Din was garnered from cell phone intercepts. These may have been provided to the Tribunal by the NSA (Hizballah is a high priority collection target for the US Intelligence Community).