Pakistan's former Prime Minister, Benazir Bhutto, has been assassinated at a rally in Rawalpindi, just a fortnight before a planned general election. It was the second attempt on her life since her return from exile in October. It's already leading to violence and tension on Pakistan's streets, as her angry supporters blame the government of President Musharraf for letting the assassination happen.
The first thing to say is that the direct involvement of senior figures in the Pakistani authorities, and the notorious ISI intelligence service, seems unlikely. Although they'd fallen out since her return in October, some kind of deal allowing Mr Musharraf to continue as President, with Ms Bhutto once again as Prime Minister, appeared a likely outcome from the election. The fact the attack took place in the usually-secure Rawalpindi is a particular embarrassment for Mr Musharraf, although it does suggest the killer or killers may have been helped by low-level figures within the army or the ISI. Musharraf may not have liked Benazir much, but he didn't want her dead, not that that will prevent conspiracy theories springing up in the coming days.
It's far more likely the killer or killers were Islamist extremists. Ms Bhutto was a hate figure for Pakistan's many religious zealots, both because of her own western education and modern outlook, and because she offered the prospect of a secular and relatively liberal government of Pakistan. Her killing makes it more likely the election will be postponed, and certainly raises the possibility of a more chaotic future in Pakistan, which would only help the Islamists.
But today could just be the event that unites Pakistan against the extremists - if the horror felt across Pakistan lasts into the weeks and months ahead. The beneficiary may be Benazir's old rival and sometime ally Nawaz Sharif. If he can tap into the support enjoyed by her PPP party, he may take her place as Pakistan's most popular politician. He may be unpalatable to Mr Musharraf (who ousted him as Prime Minister in a military coup) and a far more conservative figure than Ms Bhutto, but Pakistan's American allies will surely find him the least-worst option in a country increasingly threatened by the rise of Islamist extremism. With the death of one courageous politician, another must now step forward for the sake of Pakistan's future.