Chow Hang-tung, a barrister and one of the leaders of HK Alliance, the Tiananmen Square candlelight vigil organiser, arrived at police headquarters to submit a letter saying the group won't provide materials requested by the national security police unit. The Alliance is being investigated under the powerful national security law that China imposed on Hong Kong to stamp out dissent after huge and often violent democracy protests broke out two years ago. IMAGES AND SOUNDBITES