A suicide bomber has killed at least 43 people and injured more than 40, on the western outskirts of the Iraqi capital, Baghdad.
The Sunni militia fighters, known as Sahwa or Awakening Councils, were once allied with al Qaeda, but turned against the militant group in 2006.
They had been waiting to be paid at an army base when the suicide bomber attacked them.
The BBC reports that the Sahwa have been credited with helping to reduce the overall levels of violence in Iraq since they joined the US military and government forces in the fight against al Qaeda.
But they have frequently been targeted by militants and have recently complained about harassment from government troops as a political vacuum continues following inconclusive elections in March.