Four South Koreans involved in the lucrative business of casino 'junkets' escaped death yesterday after a hand-grenade booby trap was set at their flat. The four were saved by a visitor who spotted the hidden booby trap when she arrived at about 7.30 am. The woman, believed to be another South Korean, shouted at the four to stay inside and called the police, who defused the device. Macau is home to a sizeable number of North Koreans, but police said it was unlikely to be a political dispute. Sources said the four were involved in 'junkets' - free trips for high-rolling gamblers who buy stacks of chips in advance. About 60,000 South Koreans visited Macau in 1996, the fifth largest group of visitors, according to Macau government figures. Although tour operators have reported threats and sporadic violence over commissions, this is the first time operators of tours for South Koreans have been targeted in such a manner. The hand-grenade was similar to those used by the People's Liberation Army, although a Macau Security Force spokesman declined to give exact details. It was taped to the door frame, with fishing line tied to the pin. Anyone leaving the flat would dislodge the pin, triggering a blast, police said. The four residents of the flat were last night being questioned by police.