At the end of Nottingham Forest's 3-2 victory their striker Nathan Tyson picked up a corner flag bearing the Forest emblem and waved it in celebration in front of the 4,376 travelling Derby fans in the Bridgford End, sparking ugly scenes in a week already marred by crowd violence before, during and after the east London derby between West Ham and Millwall.
Rams players Dean Leacock and Gary Teale confronted Tyson for inciting the Derby fans behind the goal, sparking the majority of the rest of both squads to rush over and get involved as stewards fought to keep supporters off the pitch.
An FA spokesman told Press Association Sport: "The FA can confirm that we will be investigating the incidents at Nottingham Forest today as a matter of urgency."
Forest boss Billy Davies was determined not to let the melee overshadow his side's success over their local rivals, their first since March 2003, as Radoslaw Majewski and Dexter Blackstock grabbed their other goals.
He said: "I didn't see what happened but it doesn't take the edge off the win at all.
"We have beaten Derby for the first time in over six years and the fans should enjoy it because it was a wonderful performance and result.
"I thought we were magnificent in the first half and we absolutely pummelled them."
The Forest boss added: "I didn't see the incident but if that's what Nathan has done then I will look at the video.
"I remember the last time Derby were here and Robbie Savage was waving a Derby scarf around.
"There was very little made of that and we hope it will be the same for this.
"We never complained about it or made a fuss because we understood that Derby should enjoy the victory they had here."
Derby manager Nigel Clough also missed the incident involving Tyson.
He said: "I was in the dressing room, which I personally think all managers and benches should be at the final whistle.
"I'm sure the referee was out there and the officials were out there so I'm sure they've seen it and it will all be included in their report."
On the game itself, Clough felt his side were unlucky to be returning to Derbyshire empty-handed.
Forest were 3-0 up at the break but two deflected goals early in the second half from Miles Addison and Jake Livermore, his first since joining the Rams on loan from Tottenham, set up a thrilling finale.
"I thought we were a little unfortunate to come in 3-0 at half-time having created two one-on-one chances ourselves and had a goal disallowed," said Clough.
"I can't remember our keeper having a save to make strangely enough, it wasn't as if our goal was getting peppered and he was pulling saves off left, right and centre.
"We restored a little bit of pride in the second half with an exceptionally good performance and there are positives we can take from the game.
"We had a go in the second half and if you are going to go down then you go down fighting and that's what we did."