Croatia qualified for the World Cup with a 2-0 win over Iceland on Tuesday. Video footage shows Simunic taking a microphone to the field after the match and shouting to the fans: "For the homeland!" The fans respond: "Ready!"
That was the war call used by Ustashas, the Croatian pro-Nazi puppet regime that ruled the state during World War II when tens of thousands Jews, Serbs and others perished in concentration camps.
The Australian-born Simunic, who faces disciplinary action by FIFA, defended his action.
"Some people have to learn some history. I'm not afraid," the 35-year-old defender said. "I did nothing wrong. I'm supporting my Croatia, my homeland. If someone has something against it, that's their problem."
The same chant coupled with the Nazi salute has often been used by Croatian fans in the past. FIFA and UEFA have often sanctioned the Croatian Football Association because of their behavior.
Two weeks ago, FIFA fined Croatia $38,000 for incidents, including fans making salute gestures during its previous match in Zagreb, a 2-1 loss to Belgium on Oct. 11.
FIFA said Wednesday it was considering disciplinary action.
"We are in the process of analyzing the different match officials' reports and gathering information," FIFA said in a statement.
Simunic faces a possible suspension that could apply to matches in Brazil at the World Cup.
At the 2006 World Cup, Simunic was the player who received three yellow cards in one match against Australia before being sent off.