Police have ordered an internal investigation into a Wan Chai bar brawl involving five senior off-duty expatriate officers and the son of an African diplomat. Police sources said officers from Hong Kong Island Regional Crime Unit had been asked to prepare a 'secret brief' on the incident in the Devil's Advocate bar in Lockhart Road early last Saturday. They have taken over the separate criminal investigation from Wan Chai district crime squad. The incident left 35-year-old Michael Armstrong, Assistant Divisional Commander for Castle Peak, needing hospital treatment after he was hit over the head with a bottle in the street outside the bar. Nigerian Consul-General Ebenezer Olusanmokun, whose son Oluwaenia, 20, was one of two men arrested, claimed his son was singled out and victimised by the officers, who allegedly insulted him and quarrelled with him over a woman. Mr Olusanmokun has lodged a complaint with the Protocol Division. The division has refused to comment. Another man, 19, was also arrested, and both were released on $1,000 police bail hours later. They are due to report back to police today. The internal investigation will focus on which officers were in the bar, how long they were there, how much they had had to drink, and who did what. The South China Morning Post has been told that as well as Mr Armstrong, the following were in the bar: David Williams, Senior Inspector of the Organised Crime and Triad Bureau; Peter Burbidge-King, District Commander of Kowloon City; David Cameron, Assistant District Commander of Kowloon City; and Ian Seabourne, Deputy Regional Commander of Kowloon West. The reason for the sensitive internal investigation is not clear, but the Hong Kong Island detectives will pass the report direct to the force's director of operations, Senior Assistant Commissioner of Police Ng Wai-kit. A police source said: 'The director of operations wants to know exactly what happened, so a secret brief is being prepared.' It is understood that a number of the officers who were in the bar have given statements to officers investigating the incident. Before the incident, the off-duty officers had been celebrating the work of the 18th-century Scottish poet Robert Burns at police headquarters in Arsenal Street. Celebrations for 'Burns Night' are usually held around the end of January. It is the second time in a month that police have become involved with the diplomatic corps. Last month Vietnamese Consul-General Nguyen Viet Hung was released unconditionally when he invoked diplomatic immunity after a woman accused him of indecently assaulting her in a Causeway Bay street. Mr Hung denies the accusation.