Inigo Cabacas died after sustaining injuries caused by a rubber bullet after watching his side's last eight draw with the Gelsenkirchen outfit in Europe
The San Mames clash was followed by disturbances in the streets, and Inigo Cabacas, a 28-year-old fan was shot in the head before being rushed to hospital, and passing away four days later.
An autopsy revealed that Cabacas' death was caused by a rubber bullet, and Ares has promised an inquest into whether police acted properly in dealing with the crowd trouble.
"The autopsy shows that a rubber bullet was the cause [of death]. Therefore we have to establish whether protocols were adhered to or not," Ares told a press conference.
"It is unacceptable that something like this should happen in a democracy in 2012. It is essential to establish what happened and assume the consequences," Antonio Basagoiti, the regional Popular Party (PP) leader added.
Athletic Bilbao overcame the German side 6-4 on aggregate and will face Sporting Lisbon in the Europa League semi-final.