US and Iraqi investigators in Baghdad are focusing their attention on two possible culprits for Sunday’s bombings, which left more than 155 people dead.
They believe the attacks bear the hallmarks of al-Qaeda in Iraq, and Baathists who were loyal to the late former president, Saddam Hussein.
A third possibility - elements of the Awakening Councils, former Sunni Arab insurgents now supposedly co-opted by the Iraqi government - has been suggested by the Arabic-language newspaper, al-Quds al-Arabi.
However, a Pentagon spokesman has dismissed this as being unlikely because of the huge scale of the simultaneous double bombing.
‘Gaps in security’
Picking over the rubble left by yesterday’s bombs, US military forensic investigators and bomb-disposal specialists are working side-by-side with their Iraqi counterparts.
Officials say the early indications are that only two groups could have been behind such mass, indiscriminate slaughter - al-Qaeda in Iraq or the diehard loyalists of the Baath Party.
Both groups have been severely weakened in the past two years and have little popular support.
But they still have access to explosives and a seemingly inexhaustible supply of volunteers for suicide missions such as this.
They also share an almost pathological contempt for the elected government of Prime Minister Nuri Maliki, which they accuse of - variously - being the puppet of Iran or the United States.
On Monday, a Pentagon spokesman said there was some evidence of past collusion between the two groups.
There are also suspicions that the bombers may have had help from someone inside Iraq’s security forces to get their deadly cargoes through numerous checkpoints.
“The bombers took advantage of the recent changes in the security apparatus,” said Sajjan Gohel of the Asia Pacific Foundation.
“With the Iraqis now in charge of security in Baghdad instead of the Americans, the bombers may have been able to exploit certain gaps in security.”
So what would the bombers hope to gain from murdering so many people?
Their ultimate target was, of course, not so much the individuals who were killed, but the Iraqi government itself.
By showing, in such a spectacular, headline-grabbing way, that the capital is still not safe, the attackers would be hoping to reverse the gradual process of “normalisation” that has been under way for nearly two years.
Iraq is scheduled to hold elections in January and al-Qaeda has always made it clear that it considers Western-style democracy abhorrent and incompatible with Islam.
Surviving former elements of Saddam Hussein’s regime, some of whom now live in Syria, are also unwilling to accept the status quo of an elected Shia-dominated government in Baghdad, which has good relations with both Washington and Tehran.
In their eyes, anything that helps set that back, or makes Iraq harder to govern, is worth doing, however many innocent lives are lost.