By Roger Hardy
BBC Middle East analyst
It was by all accounts an inside job.
Some Sunnis in Anbar have joined forces with the Americans
As members of the Issawi tribe gathered for a celebration, a teenaged boy arrived with a box of sweets and detonated an explosives belt.
At least four people were killed, including a leading member of the local Awakening Council - a Sunni group in Anbar province which has joined forces with the Americans to fight al-Qaeda militants.
The suicide attack, in a village near Falluja on Sunday, was one of a string of recent attacks in Sunni areas.
But what was noteworthy about this one was that it revealed the depth of disunity among tribesmen the Americans are relying on in their efforts to keep Anbar stable.
Reports from the area suggest the Issawis, a large and important tribe, are split. A majority supports the local Awakening Council, while a minority is sympathetic to al-Qaeda.
The Americans now find themselves caught up in a complex web of tribal feuds and rivalries
Local people say the young teenaged bomber's father was a senior figure in al-Qaeda.
It seems his family sent him off on a suicide mission, hoping his tribal links would help him get access to the celebration.
In response to Sunday's attack, recrimination between the two factions has intensified, with tribesmen reportedly burning down the house of the bomber's family.
The incident has serious implications for US strategy.
It underlines the difficulty the Americans face in providing adequate security for their Sunni allies - and the constant danger of infiltration by militants.
It also suggests the Americans now find themselves caught up in a complex web of tribal feuds and rivalries.
Loyalty 'a vital prize'
Reducing the violence in Anbar was one of the success stories of 2007, and the Awakening Councils have been an important ingredient in that success.
But Sunday's attack will increase the foreboding of the Shia-dominated government in Baghdad, which all along has been distinctly uneasy about the US policy of arming and funding Sunni allies.
Shia politicians have warned that the loyalty of the tribes cannot be taken for granted.
In the bitter war between US forces and al-Qaeda, that loyalty is a vital prize.
The battle for Sunni hearts and minds is far from over.