The boss of a drug-treatment prison whose perceived management style allegedly helped spark a riot by 400 inmates will be transferred to another correctional institution on Monday. The move comes as prison chiefs plan additional security measures after a June riot at the Hei Ling Chau centre, off Lantau Island. The measures will include installing fences at assembly points to allow better control in emergencies. Superintendent Mui Kwok-keung was said to have been one of the targets during the riot on June 4 because of his perceived preferential treatment of Vietnamese inmates. The riot was sparked by a verbal dispute between local and Vietnamese inmates. Prisoners laid siege to a cell where the 13 Vietnamese inmates were taking shelter and threw burning objects into a dormitory. The rampage left 35 people injured and damage estimated at $10 million. Police are still investigating the case and 70 inmates face charges of rioting. Mr Mui, who lost valuables worth $100,000 from his quarters during the riot, had said he was determined to stay in his post at Hei Ling Chau, which he took up last October. But Mr Mui, who said the allegations against him were unfair, will be transferred to Sha Tsui Detention Centre, on Lantau, from Monday. He is on leave and could not be reached for comment yesterday. His duties have been taken over by Superintendent Wong Hon, formerly chief of the Sha Tsui centre, a minimum-security institution for young offenders. The Correctional Services Department would not comment on whether Mr Mui's transfer was related to the riot. 'We do not comment on individual transfers,' a department spokesman said. It is understood the alleged preferential treatment had been investigated in the department's internal inquiry, which was submitted to Secretary for Security Regina Ip Lau Suk-yee yesterday. Mr Mui was temporarily transferred to headquarters shortly after the riot, with the department saying his legal expertise was needed for a project. He was the acting senior superintendent - overseeing the three correctional centres on Hei Ling Chau island - when the riot broke out. Mr Wong said some security improvements had been worked out at the Hei Ling Chau centre, including replacing old gates and locks. He said they were considering ways to store liquefied petroleum gas cylinders more securely. Inmates had armed themselves with the cylinders in the riot. 'We'll conduct regular searches of inmates' cells to make sure they do not possess any weapons,' Mr Wong said, adding that they had also reminded contractors to ensure they had packed away all tools and materials on building projects at the centre.