By Frank Gardner
BBC Security Correspondent
Ms Bhutto made many enemies in Pakistan
Who killed Benazir Bhutto?
The actual assassin, when and if his identity is discovered, was doubtless someone most people will never have heard of.
What matters is who sent him and why.
In the fevered speculation now gripping the streets of Pakistan there are essentially two conflicting theories.
The first theory, the one espoused by the Pakistani government, is that al-Qaeda or the Taleban, or even both, were the killers.
Certainly the method of attack - a suicide bombing in a crowded place - is al-Qaeda's favoured modus operandi, although the assassin was taking no chances by also opening fire with a pistol just before he blew himself up.
Both al-Qaeda and the Taleban had every reason to want Ms Bhutto dead.
Anathema to Islamists
As a secular, Western-educated, female politician with close ties to Britain and the US, she represented much that is anathema to Islamist extremists.
She also publicly criticised President Musharraf for not doing enough to curb their power in Pakistan and she accused the Inter Services Intelligence (ISI), Pakistan's powerful military intelligence arm, of harbouring Islamists with sympathies for al-Qaeda.
Much will depend on whether Ms Bhutto's assassin was sent by someone inside or outside mainstream Pakistani society
If al-Qaeda was behind this assassination its normal tactic would be to wait for a while to encourage confusion and then release a carefully prepared statement on the internet, laced with religious phraseology, praising the assassin and listing its reasons for the attack.
The conflicting theory, taken up by many of Ms Bhutto's supporters, is that the government of President Musharraf is to blame.
Specifically, they blame elements in the ISI who they believe felt so threatened by Ms Bhutto's potential return to power that they took drastic action.
Inside or outside?
Despite the passion with which some hold this conviction there is no independent evidence to support this and in the absence of a truly transparent investigation the truth may never come out.
Woven into Pakistan's complex political fabric are a number of militant Islamist groups that belong to neither al-Qaeda nor the government yet have ties with one or the other, or even both.
For years the ISI supported the Taleban in Afghanistan and for years it supported Kashmiri separatist militants.
Musharraf claims to have rid the army of its Islamist elements
Although President Musharraf has gone to some lengths to convince Washington that he has purged the ISI and the military of anyone with links to terrorism, there are many who suspect some of the old ties have yet to be broken off completely.
In the coming days and weeks Pakistan's internal security situation will depend in part on whether Benazir Bhutto's assassin was sent by someone inside or outside mainstream Pakistani society.
If the culprits are found to be from outside, such as al-Qaeda or the Taleban leadership, then this could possibly have a unifying effect on Pakistanis, most of whom are appalled at this kind of extremist violence that has destroyed a national figure.
If however the culprits represent a recognisable faction from within Pakistani society - and especially if that faction were connected to the government - then there is a risk of far greater unrest to come.