Butterflyer Ian Crocker dove in before Scott Usher completed his breaststroke leg, resulting in the DQ. It was the first time any U.S. relay team failed to reach a final in world championships' history.

''Just an unlucky exchange,'' said Neil Walker, who swam the anchor freestyle leg.

Ryan Lochte led off on the backstroke.

Crocker was back in the pool for the first time since losing the 100 butterfly to Phelps the night before. Phelps' victory was his sixth gold medal of the meet, tying Ian Thorpe's record from 2001.

Walker attributed Crocker's mistake to ''a little bit of overexcitement.''

''Ian had such an awesome race last night with Michael Phelps. To get back up the next morning is tough to do in the morning relay,'' he said.

The Americans completed their exchange in 0.04 seconds -- one-hundredth of a second outside the allowable time, so no protest could be lodged.

''That's just incomprehensible, that amount of time,'' Walker said. ''It's so small that you can never really plan that.''

The result provoked instant reaction from other countries. Members of Singapore's relay pointed at the 'DSQ' listed next to the United States on a television monitor and smiled.

Walker spoke on behalf of the relay team, with Crocker walking by reporters in silence.