I flew ARN-HEL-DFW-MCO with Finnair operating the first two flights and the third flight operated by American Airlines. The connection was delayed by 4 hours meaning I arrived over 4 hours late at my final destination.

I understand that EU261 is valid if it is all on a single ticket (as this was). I wrote to Finnair but they said I need to claim from AA. The booking was made with Finnair with all flights having an AY code. Is Finnair correct? or is it Finnair who I need to pursue?

Thanks in advance