Aid agencies paid Somalia's al-Shabab militants for access to areas under their control in the 2011 famine, according to a joint report by two think tanks. In many cases al-Shabab insisted on distributing the aid and kept much of it for itself, the report says. Some of the groups are still paying al-Shabab to operate in the large parts of Somalia it still holds, it adds.
More than 250,000 people died during the famine, caused by a drought. The disaster affected more than 13 million people across the Horn of Africa and triggered a major refugee crisis, with hundreds of thousands of Somalis fleeing the rural areas controlled by al-Shabab.
The militant group had banned several international aid agencies. Many people walked over the border to camps in Kenya and Ethiopia or to Somalia's government-controlled capital, Mogadishu.
'Sign a pledge'
The report - by the Overseas Development Institute and the Mogadishu-based Heritage Institute for Policy Studies - details how al-Shabab demanded from the agencies what it described as "registration fees" of up to $10,000 (£6,100).
It gives one example of al-Shabab diverting food aid in the town of Baidoa, where it is reported to have kept between half and two-thirds of food aid for its fighters.
Al-Shabab, which is linked to al-Qaeda, developed a highly sophisticated system of monitoring and co-opting the aid agencies, even setting up a "Humanitarian Co-ordination Office".
Aid groups had to deal with this office, even though they risked legal problems by doing so because of counter-terrorism laws in other states which forbid engagement with groups like al-Shabab.
The report says agencies who worked in al-Shabab-held areas had to complete special forms and sign a pledge saying they would refrain from certain social and religious activities.
It also describes how al-Shabab gave people extra food if they spied on the aid groups. Some agencies were banned outright by al-Shabab, including most UN agencies, while others withdrew because of the demands.
The report does not specify which agencies agreed to pay fees to al-Shabab. The UN declared the famine over in February 2012.
Over the last two years, al-Shabab militants have been driven out of Somalia's major towns and cities by pro-government forces and a UN-mandated African Union force of some 18,000 soldiers. But the Islamist group still controls many towns and rural areas of southern Somalia.