Deutsche Boerse and Nasdaq are being investigated as part of an European Union probe into possible antitrust violations in financial derivatives.
A spokesperson for Deutsche Boerse said on Tuesday the German stock exchange operator had been searched as part of the investigation, while a representative for Nasdaq said it had also been approached. Both companies said they were cooperating.
The European Commission said on Monday it had conducted unannounced inspections at financial services companies in two EU member states over possible breaches of antitrust regulations, but did not specify which firms or how many.
"We do not comment on ongoing investigations," the Deutsche Boerse spokesperson added.
A Stockholm-based spokesperson for Nasdaq declined to comment on which country or countries Nasdaq had been contacted in.
"We are aware of an investigation initiated by the European Commission involving the derivatives market. Nasdaq is committed to fully cooperate with the European Commission and support the relevant authorities with the investigation," the person said.
Euronext, which runs stock markets in various cities including Paris and Amsterdam, said it was not the subject of "any inspections or investigations" by the Commission.
Unannounced visits are a preliminary step in probes into suspected anticompetitive practices and do not mean the firms are guilty. There is no legal deadline to finish inquiries into such conduct, according to the EU.
If the inspections reveal violations of EU antitrust laws, it could trigger formal investigations and significant fines for the companies involved.