Authorities intensified their search for Tibetan protesters yesterday, releasing photos of wanted suspects who were captured on video cameras in the worst rioting Lhasa has seen for almost two decades. Police in the Tibetan capital called on the public to provide clues leading to the detention of 21 rioters. Their pictures have been cropped from video footage and released publicly. A hotline number has also been set up and publicised by China's major news portal and television stations. A Xinhua news report said two suspects had been detained by police and another had surrendered on Wednesday night. International rights groups said police had conducted house-to-house searches in recent days and had detained hundreds of people. They were concerned about more detentions and possible torture. Lhasa prosecutors announced they would file criminal charges against 24 people for 'endangering national security' and other charges. Peaceful demonstrations by monks in Lhasa on March 10, marking the anniversary of a failed uprising against Beijing's rule in 1959 which forced the exile of the Dalai Lama, escalated into violent riots on March 14. Protests have since flared up in neighbouring Qinghai , Gansu and Sichuan . Beijing said 13 civilians in Lhasa had been killed in the riots and more than 300 injured. Tibetan exile groups said 99 people had been killed, 80 in Lhasa and 19 in Gansu. But the death toll is difficult to confirm because foreigners and journalists have been banned from Tibet and authorities have tried to block them from entering neighbouring provinces such as Gansu and Sichuan. The authorities had denied using lethal weapons or opening fire on the rioters in Lhasa and insisted 'maximum restraint' had been exercised in Gansu. But they admitted on Thursday night that four rioters had been shot and injured in Aba county, Sichuan. In a correction of a report, Xinhua said police had opened fire in self-defence, wounding four people who were trying to seize ammunition from a police station. But rights groups who released photos of dead bodies on Tuesday said at least 13 protesters had been killed in clashes with security forces in Aba on Sunday and Monday. 'After the warning shots were ignored, the police officers were forced to open fire in self-defence in accordance with the law when their lives were put in danger and with the mobs trying to grab guns the officers took from the safe,' Xinhua reported. It said the four wounded rioters left with their accomplices and that the police were verifying the details.