"Twenty years on from the tragedy at the Argentine Jewish Charities Federation (AMIA), I want to express my closeness with the Argentine Jewish community and all the victims' families, whether Jewish or Christian," the pope said in a recorded message played at commemorations of the anniversary.
"I've said before that Buenos Aires is a city that needs to weep, that still hasn't cried enough," said Francis, who served as archbishop of Buenos Aires before becoming pope and was auxiliary bishop of the city when the bomb exploded at the AMIA on July 18, 1994.
"That's why it has been so difficult to find the path to justice, to face the debt this tragedy has left with society.... Together with my closeness, my prayer for all the victims, I would also like to send my wish for justice. May justice be done," Pope added.
Argentina charges that Lebanese Shiite movement Hezbollah carried out the attack under orders from Iran, which Tehran denies.
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Since 2006, Argentine courts have demanded the extradition of eight Iranians, including former president Akbar Hashemi Rafsanjani, former defense minister Ahmad Vahidi and Mohsen Rabbani, Iran's former cultural attache in Buenos Aires.
The vice president of the AMIA, Ralph Thomas Saieg, criticized Argentine authorities Friday for "the little that has been done" to find those responsible.
After sirens rang out to mark the moment the attack occurred, at 9:50 am, the names of the victims and their ages when they died were read out at the AMIA.
The pope's video message, recorded on the cell phone of the director of the Latin American Jewish Congress, Claudio Epelman, was then projected on a screen for hundreds of people marking the anniversary, though no government officials were present at the ceremonies.