In one of the stranger plays of Sunday's game, Panthers' running back DeAngelo Williams scampered 30 yards down the sideline for a touchdown, as Redskins defenders pulled up and watched.
The reason was an inadvertent whistle by an NFL line judge, who thought he saw Williams step out of bounds as he neared the sideline.
"I would've pushed him out of bounds if there wasn't a whistle," Riley told reporters after the game. "From the angle I was coming from, I couldn't see if he stepped out or not. So when I heard the whistle I stopped.
"I don't know who's at fault, but I heard it."
Officials huddled after the play and upheld the touchdown, giving the Panthers their first lead of the game.
After the game, NFL referee Carl Cheffers answered questions from a pool reporter, explaining his decision not to honor the whistle.
"We just felt when the whistle blew, that the player would have already scored a touchdown," he said. "We tried to piece together if we had to spot. By rule, we would have to put him down when the whistle blew. We tried to decide where that spot would be, and we felt that spot would be in the end zone.
"By the time the whistle blew, he had already crossed the goal line."
Riley disputes the timing of the whistle, and TV microphones picked up an audible whistle when Williams was between the 20 and 15-yard lines.
Riley said after the game that he did not want to subject his team to a 15-yard personal foul for a late hit after the whistle.
"I don't want to push him out after the whistle and get a 15-yard personal foul," Riley said. "I hate it had to happen like that because it makes me look like I wasn't hustling. I always play 100 percent, every snap until the whistle.
"That [whistle] takes a touchdown off the board. If we stop them and get a field goal it's a whole different game."
After the game, Redskins head coach Mike Shanahan told the media he would have appreciated the opportunity to review the play, rather than making the ruling by committee on the field.
"You would think they would [review]. I didn't know that was part of the situation at the time," Shanahan said, who said officials told him the play was being reviewed for a possible holding call.
"I didn't know about the whistle--they didn't say anything about that. The guy put his hands up in the air and that was clarification that they gave me that that was what they were talking about."
Mike Pereira, the former NFL Vice President of Officiating and current FOX NFL analyst, tweeted his disapproval of the call during the game, saying:
"In WSH/CAR even though he didn't step out of bounds the official should've admitted he blew the whistle and ball should have been dead there."
Cheffers said after the game that the offense has the choice during an inadvertent whistle to take the result of the play or replay the down. The defense does not have that luxury.
"They’re obviously going to want to take the result of the play as a long gainer," he said. "The spot was going to be important, and that’s why we were trying to piece together where that spot would be.
"[Touchdown] was our decision, and that’s why I announced that the ruling on the field is touchdown."