Spain's King Juan Carlos and Queen Sofia led a memorial service outside Madrid's Atocha train station Tuesday to mark the fourth anniversary of the train bombings that killed 191 people.
The royal couple placed a wreath at the memorial and observed several minutes of silence.
The bombs tore apart four commuter trains that were headed for the Atocha station on March 11, 2004.
Al-Qaida-linked terrorists claimed responsibility for the attack, calling it a response to former Prime Minister Jose Maria Aznar's support for the war in Iraq.
A Spanish court convicted 21 suspects for their roles in the bombings.
Analysts say the attack was partially responsible for the Socialists and Prime Minister Jose Luis Rodriguez Zapatero winning parliamentary elections three days later.