Two Swansea City players will be interviewed by police after parading anti-Cardiff obscenities after their Football League Trophy final victory.
Striker Lee Trundle and defender Alan Tate carried a Wales flag daubed with an offensive slogan, and Trundle wore a T-shirt with an insulting cartoon.
Swansea apologised, saying players had become caught up in the atmosphere at the Millennium Stadium in Cardiff.
The Football Association of Wales is investigating the "insulting" images.
It happened as Swansea players were celebrating their 2-1 victory over Carlisle United on Sunday.
Although Cardiff City were not involved in the game, there is a history of tension between supporters of the two south Wales clubs, which are 40 miles apart.
The actions of the Swansea players involved have been criticised by South Wales Police, the Football League and by stadium management.
Trundle has not commented on his actions but defender Tate said: "I'm getting interviewed by the police this week but that's all I'm saying on the matter at the moment."
On Monday afternoon, Swansea issued a statement which apologised "for any distress caused" saying the players involved had become caught up in the atmosphere".
Lee Trundle wore a T-shirt with an anti-Cardiff cartoon
The statement read: "Swansea City Football Club would like to apologise unreservedly for any offence caused, particularly to Cardiff City Football Club and its supporters.
"In the euphoric atmosphere following the presentation of the trophy certain players, caught up in the moment, picked up articles thrown from individuals in the crowd without realising the offensive nature of some of these items.
"The players concerned, Alan Tate and Lee Trundle, wish to personally apologise to all those offended by their actions. Both players stress that no malice was intended.
"We would like to thank the people of Cardiff for their warm welcome and their help in making the day a memorable one."
Millennium Stadium chief executive Paul Sergeant said the flag had been confiscated by league officials and was being held by stadium staff.
Mr Sergeant told BBC Wales it was the first time something like this had happened at the stadium in more than 200 games and said it was "totally uncalled for".
South Wales Police have launched an investigation and Chief Supt Bob Evans said: "We've had a number of complaints - many from Swansea fans, it must be said - and are therefore obliged to launch a full investigation.
"It's highly likely that we'll be speaking to a number of players as soon as possible in relation to this matter, and also evidence such as CCTV will be closely examined.
"I want this matter cleared up as quickly as possible because Swansea could be playing in Cardiff again in a few weeks' time if they get to the League One play-off final."
A statement from the FAW said it was collating evidence for possible misconduct charges "against certain individuals and the club".
It added: "The images paraded by Swansea City players were of an extremely offensive and insulting nature and such behaviour is totally unacceptable."