Leading article: Yet more bloodshed on a country's chequered passage to democracy

The situation in Pakistan was already fraught with uncertainty even before Benazir Bhutto's tumultuous homecoming. But the attack on her convoy as it made its slow progress through Karachi in the early hours of yesterday has made it infinitely more dangerous. The death toll stands at 130; as many as 500 people were injured and, while Ms Bhutto herself was unhurt, the joy that attended her return was transformed within seconds into grief and apprehension.

What happens next depends in part on who ordered the blasts, and why. Was the intention to kill the returning former prime minister, or to intimidate? Was it to destabilise President Pervez Musharraf by provoking civil strife or perhaps to strengthen him by eliminating the leader of an alternative power base?

Some singled out al-Qa'ida or the Taliban. Ms Bhutto's husband, speaking from Dubai, blamed members of Pakistan's government and intelligence agencies. This would suggest that there were sharp divisions in the leadership or that the deal facilitating her return was concluded in cynical bad faith. In that case, the former prime minister was lured into a trap.

Ms Bhutto herself said that supporters of the late military ruler Mohammed Zia ul-Haq were responsible. It was General Zia who overthrew her father and had him hanged. His death in an unexplained plane crash opened the way for Ms Bhutto's first return from exile and the first of her two stints as prime minister. Plenty of bad blood flows between the two clans.

Her theory leaves her deal with President Pervez Musharraf intact. But this, too, raises as many questions as it answers. So far as is known, Ms Bhutto has been permitted to return and spared corruption charges in the expectation that her People's Party will fight, and win, parliamentary elections which must be held before mid-January. With Ms Bhutto ensconced as Prime Minister, Mr Musharraf would resign from the military and remain in office as a civilian. Thus would Pakistan make a smooth, if constitutionally questionable, transition from military rule to democracy.

Except that none of these elements can be guaranteed. Mr Musharraf's election victory earlier this month is being challenged in the country's Supreme Court. He cannot be sworn in until the case is over – and Pakistan's courts are no longer the rubber stamp they used to be. Nor is an election victory for Ms Bhutto's People's Party the sure thing it might once have been. The terms under which Ms Bhutto has returned have alienated many supporters, who regard them as having been cooked up by the US and Britain to keep Mr Musharraf in power. There are also doubts about the amnesty she has supposedly been granted. It depends not only on Mr Musharraf's election victory being upheld, but also on those newly "unreliable" courts.

Even if all these elements are eventually in place, however, the quality of Pakistan's democracy will still be compromised so long as another former prime minister, Nawaz Sharif, leader of the Pakistan Muslim League, remains in enforced exile. His attempted return six weeks ago ended in summary deportation, even though it had been sanctioned by the courts. Without his participation, parliamentary elections would be flawed.

Ms Bhutto was admirably defiant yesterday, describing the attack on her convoy as "an attack ... on the very unity and integrity of Pakistan" and pledging to continue her struggle to restore the country's democracy. Now that it has been so conclusively shown that the regime cannot guarantee her safety, however, it must be asked how effectively she will be able to campaign, if at all, and how secure any future government might be. Another element of instability has been added to an already highly volatile mix.