American swimmer Dana Vollmer took the gold medal in the women's 100-meter butterfly in world record time Sunday at the London Summer Olympics.

Vollmer, the reigning world champion, swam the 100 meters in just 55.98 seconds. China's Lu Ying won the silver and Alicia Coutts of Australia took the bronze.

In the men's swimming 4×100 relay, France avenged its narrow loss to the Americans four years ago by winning the gold this year. France's Yannick Agnel pulled ahead of American Ryan Lochte in the final lap. The United States won the silver and Russia the bronze.

Also Sunday, North Korea won its first two gold medals of the games. An Kum Ae won the women's 52-kilogram judo competition, beating Cuba's Yanet Bermoy Acosta. Om Yun Chol won the men's weightlifting gold, followed by China and Azerbaijan.

Georgia's Lasha Shavdatuashvili won the men's 66-kilogram judo competition.

The heavily favored U.S. men's basketball team won its opening match, beating France 98-71. Both teams' rosters include a number of National Basketball Association stars.

Organizers of the London games are investigating why there have been so many empty seats in the stands at multiple sporting events in the first days of competition.

Television images showed empty rows of seats at Wimbledon, the Aquatics Centre and soccer stadiums. Angry fans have been complaining that they have not been able to buy tickets.

British Culture Secretary Jeremy Hunt said blocks of the empty seats were accredited to sponsors. He said if they were not going to show up, then those tickets should be made available to the public.