Four suspects held over the deadly terror attacks in Barcelona and Cambrils are being interrogated at Spain's National Court.

The four men, believed to be the only surviving members of a 12-man terror cell, are being questioned in the presence of court-provided lawyers, a spokeswoman for the prosecutors said.

Mohamed Houli Chemlal, 21, was the first to be interrogated.

He is the lone survivor of an explosion at a house in eastern Spain, which the authorities believe was connected to the vehicle attacks that killed 15 people and wounded more than 120.

His testimony is considered key to clarifying what motivated the group to carry out the atrocities.

Suspect Driss Oukabir was arrested Thursday in the northeastern town of Ripoll, as were two others identified by Spanish media as Mohammed Aalla and Salh el Karib.