Palestinian security agents who have been detaining and allegedly torturing supporters of the Islamist organisation Hamas in the West Bank have been working closely with the CIA, the Guardian has learned. Less than a year after Barack Obama signed an executive order that prohibited torture and provided for the lawful interrogation of detainees in US custody, evidence is emerging the CIA is co-operating with security agents whose continuing use of torture has been widely documented by human rights groups. The relationship between the CIA and the two Palestinian agencies involved - Preventive Security Organisation (PSO) and General Intelligence Service (GI) - is said by some western diplomats and other officials in the region to be so close that the American agency appears to be supervising the Palestinians' work. One senior western official said: The [Central Intelligence] Agency consider them as their property, those two Palestinian services." A diplomatic source added that US influence over the agencies was so great they could be considered "an advanced arm of the war on terror". Among the human rights organisations that have documented or complained about the mistreatment of detainees held by the PA in the West Bank are Amnesty International, Human Rights Watch, al-Haq and the Israeli watchdog B'Tselem.