A leading Tibetan scholar said he believed protests in Lhasa became violent after a security 'vacuum'. That was the period after regular police escaped attacks by rioters and before the arrival of armed police. 'Earlier in the week, I thought the Tibetan government and the troops behaved responsibly because they began negotiating with the people,' said Robbie Barnett, an expert on Tibet at Columbia University in New York. 'But unfortunately once they faced serious challenges, the local government started to lose its nerve and started to adopt a hardline approach.' Tibetans' desire for independence and radical Chinese policies were the main reasons behind the outbreak of violence and remained the key issues in Tibet, he said. Professor Barnett said Chinese authorities' encouragement of Han Chinese migration to the Tibetan region and the deprivation of any 'credible' Tibetan representation in the political arena had added fuel to the fire. 'Because there was no way for the Tibetans to lash out at the government ... they vented their anger at the Chinese people instead,' he said. Professor Barnet expressed hope that Beijing would 'reintroduce a more cautious' approach. Meanwhile, contrary to Tibetan officials' claims that troops had not opened fire on rioters, Matt Whitticase, a spokesman for the London-based Free Tibet Campaign, said sources had said Chinese soldiers had killed dozens of Tibetans gathered outside Drapchi Prison, where Tibetan political prisoners were held. The International Campaign for Tibet said that the unrest was 'a result of established policies that have suppressed religious freedom, denigrated the Dalai Lama and his authority, withheld key provisions of autonomy in Chinese law ... and the marginalisation of Tibetans. 'The only lasting solution for the Tibetan problem is for the Chinese government to react positively to the Dalai Lama's call for a negotiated solution for Tibet,' said the group's president, John Ackerly.