For 'one country, two systems' to mean anything, law enforcement in Hong Kong and the mainland must remain separate from each other. There will always be cases that require cross-border co-operation, but joint policing should follow long-agreed rules that are based on international practices. This much was agreed years ago and, in general, the arrangement works well. However, the June 16 incident involving seven mainlanders arrested on Mount Davis Road seems to be an instance where the system failed, and more answers will be needed if public concerns are to be eased. Two of the men claimed to be officers of the Guangdong Public Security Bureau, while one had a set of handcuffs. All seven men were arrested and released on bail, but prosecution is still being contemplated. Apparently, our own police department had no knowledge of the mainlanders' purpose for being here or even about their presence in Hong Kong. On the face of it, the entire operation - and the reported return of two men to the scene after their release - is in violation of rules requiring notification and the presence of Hong Kong officers any time Guangdong police come to the city for work. According to Interpol practices accepted by both sides, mainland and Hong Kong police do not enjoy any jurisdiction in each other's territory, whether it is to investigate, arrest or search premises. The laws of each side remain in force and must be obeyed by the officers, who become ordinary citizens once they cross the border. Bilateral co-operation on police work includes a 24-hour hotline and half-yearly meetings between officials at the highest levels. The possibility that the Guangdong investigators were carrying out work outside accepted channels is, as Chief Executive Tung Chee-hwa and Secretary for Security Ambrose Lee Siu-kwong said, totally unacceptable. The Hong Kong Security Bureau's pledge to seek answers from the mainland and pursue the matter if any local laws have been broken is encouraging - and has to be followed by action. If the two-way permit holders are found to have violated any laws on immigration, weapons or police work, charges should be laid. Meanwhile, silence from the mainland raises concerns that answers will be difficult to get. Stonewalling on such matters has been encountered before, but should not be accepted. In past cases where mainland officers have been accused of accompanying Hong Kong residents back over the border to collect evidence, or conducting other police work on their own, charges have been dropped because of a lack of co-operation from the mainland side. But each incident was followed by vows that cross-border work rules would be made clear to frontline officers. Weeks after the arrests, we are still in the dark about the identities of the men on Mount Davis Road and their intended business in Hong Kong. We also have no clue about whether their presence was sanctioned by officials in Guangdong. The mainland side has to be pressed for details. It may result in reminders or verbal reprimands to mainland police officers, but it will solve the mystery of how and why the rules have been ignored - and whether any laws have been broken. It could also lead to some necessary improvements in the mechanism for liaison. Cross-border co-operation on law enforcement is necessary and desirable. Joint police work has been credited with successes such as the recent reduction in triad turf-war violence in Macau. It has also helped crack important cases in Hong Kong. But to maintain the separation required under Hong Kong's Basic Law - and preserve the integrity of our legal system - contacts should be regulated by the carefully negotiated rules. There have been hints that the men on Mount Davis Road were here in pursuit of a corruption case. If the case is a genuine one, there is every reason for Hong Kong to help. It would also follow that the investigation be carried out in a transparent and legal manner. If it was not, the public deserves an explanation.