A car bomb exploded at Madrid's international airport Saturday, and Spain's government, blaming the Basque group ETA, suspended plans for peace talks with the separatists.
The blast left two people missing and 26 injured, most with damage to their ears from the shock wave.
Prime Minister Jose Luis Rodriguez Zapatero said the government would halt talks with ETA over the bombing. The group had agreed to stop attacks in its cease-fire declaration this year that was seen as the greatest hope in a decade of a peaceful end to the conflict.
"The condition for dialogue was and is the unequivocal desire to abandon violence," said Zapatero, who cut short a family holiday after the bombing. "The very grave attack today by the terrorist band ETA is radically contrary to that desire."
The prime minister had insisted as recently as Friday that he was optimistic that the cease-fire would lead to a permanent peace.
ETA did not claim responsibility for the bombing, but a man who placed a warning call before the attack said he was a representative of the group. After previous attacks, the group has sometimes waited weeks to claim responsibility.
ETA and its political supporters have been saying for months that the peace process was faltering.
The head of ETA's political wing, Arnaldo Otegi, said Saturday after the attack that he did not consider the peace process dead.
"Not only is it not broken, but it is more necessary than ever," Otegi said.
The attack early Saturday, which occurred in a multistory parking garage at the airport, halted some air traffic on one of the year's busiest travel days.
More than 1,000 pounds of explosives had been packed into the car used in the bombing, Spanish media reported. The explosion sent up a massive column of smoke and collapsed part of the building, sending rubble crashing onto parked cars.
Two Ecuadorean men believed to have been sleeping inside a parked car were missing in the rubble, officials said.
More than 800 people have died since the ETA took up arms in the late 1960s, but none since May 2003. The group had continued a series of low-level bombings until just before it declared the cease-fire in March.