United States star Landon Donovan believes his side had a late goal "stolen" from them by the referee who oversaw their 2-2 draw with Slovenia.
The US went 2-0 down but fought back before Koman Coulibaly - taking charge of his first World Cup game - ruled out Maurice Edu's 85th-minute volley.
"I'm a little gutted," said Donovan. "I don't know how they stole the goal from us. I'm not sure what the call was."
Coach Bob Bradley said: "I still don't know why the goal was disallowed."
Bradley added: "This team still understands how to fight for 90 minutes. This is something we've seen time and time again.
"It is important to understand that the players put a lot of emotion into the game and now they want a fair outcome."
The US were two goals down at half-time but made a dramatic comeback, thanks to goals from Donovan and Michael Bradley - son of coach Bob - to keep alive their hopes of emerging from group C into the last 16.
But that prospect would have been far brighter had they won the match.
The crucial incident occurred five minutes from the final whistle, when Donovan's free-kick was swung into the Slovenia area and Edu volleyed the ball into the net.
Edu ran off in celebration but the Malian referee blew for an apparent infringement as the players jostled for position in the penalty area.
The US, who drew 1-1 with England in their opening match of the tournament, protested furiously and were left questioning the decision after the game.
"It's too bad because I think that was a fair goal," said Los Angeles Galaxy midfielder Donovan. "I saw a good finish and a good goal. He wouldn't tell us what the call was.
"But I'm proud of our guys. My guess is there are not many teams in this tournament that could have done what we did."
The US, who now have two points from two games, play Algeria in Pretoria on Wednesday in their final group C match.
"We know we're good enough to play against a team like that," added Donovan.
"We can't keep putting ourselves in holes like that. We've got one more chance against Algeria, and we're still alive."
Highlights - Slovenia 2-2 USA