United States have won gold in the women's 4x400m with a time just shy of the world record, 3:16.87.
The Americans were comfortably in the lead and were about 20 metres ahead when Allyson Felix took the baton on the second leg.
Sanya Richards-Ross took the baton home with a time some four seconds ahead of second-placed Russia.
Jamaica finished just short of the Russians and took bronze with 3:20.95.
Team GB were never really in contention for a medal, although they did finish with a season's best 3:24.76.
