The director of Spain’s National Intelligence Centre, Felix Sanz Roldan, has appeared before the county’s official secrets committee to clarify his departments role in NSA surveillance of Spanish citizens.
The probe will establish whether a crime has been committed and if a formal investigation is required.
The allegations that Spanish intelligence colluded with US agents will further dent public confidence in the government.
Respected newspaper El Mundo claimed that the NSA eavesdropped on 60 million Spanish phone calls in one month alone.
The daily said the information was leaked by former NSA worker Edward Snowden.
The United States admitted that phones in Europe were monitored, but said the NSA did not act in isolation.