He said two of the men were permanent residents of the United States: Ha Lai Ninh, 38, from Texas, and Thai Dac Phat, 39, from Florida. The other two were Vietnamese.

The police officer said the two returned to Vietnam recently and began running the betting ring on Friday when the World Cup kicked off in Germany.

Betting on soccer matches is illegal but rampant in soccer-crazed Vietnam.

MUNICH PROSTITUTION RAID: A raid in Munich's red-light district found no signs of women forced into prostitution for the World Cup, although police said Monday they found significantly more prostitutes at work than usual.

Officers raided 48 brothels and checked six streetwalking zones in the Bavarian capital on Saturday night, a day after the tournament opened.

A German man who had brought a 19-year-old Ukrainian woman to work at a brothel was briefly detained on suspicion of human trafficking. He was released after police found no evidence she had been forced into prostitution, spokesman Andreas Ruch said.

Police found that "all the businesses that were checked had increased noticeably the number of prostitutes working for them during the World Cup," Ruch said, estimating the number of prostitutes working in Munich has risen to 800 from the usual 500.

However, "the increase in guests that the red-light scene was hoping for so far has failed to appear," Ruch said.

Before the World Cup, anticipation of a sex-trade boom raised fears of an increase in forced prostitution, with an estimated 40,000 women from poorer Eastern European countries forecast to head to Germany for the tournament -- some possibly against their will.

Already, the European Union estimates more than 100,000 women in Europe each year are forced into prostitution.

Prostitution is legal in Germany, with about 400,000 people registered in the trade.