A powerful car bomb exploded at Madrid's international airport on Saturday, injuring 26 people and leaving one missing in an attack blamed on the Basque separatist group ETA.

The bombing shattered a nine-month-old cease-fire and was a major embarrassment to the Spanish government, which had said it was willing to negotiate with ETA to end the decades-long conflict.

The attack at around 9 a.m. blew up a multistory parking garage on one of its busiest traveling days of the year.

After a warning call from ETA, a maroon-colored van exploded inside the garage adjoining the airport's new Terminal 4, sending a massive column of smoke into the air and collapsing part of the building, sending rubble crashing onto parked cars.

A 19-year-old Ecuadorean man, Diego Armando Estacio, believed to have been taking a nap inside a parked car, was missing among the rubble and 26 were slightly injured, mostly with damage to their ears from the shockwave of the blast.

Air traffic at Terminal 4, which is connected to the garage, was halted for several hours but returned to normal by early afternoon, said Interior Minister Alfredo Perez Rubalcaba. Flights were not affected at the other three terminals at Barajas's International Airport.

One witness, Renzo Zarzal, 28, a worker at a nearby highway toll booth, said the blast was strong enough to shake some surrounding buildings. "I was outside my booth talking to a colleague when there was a massive blast that really shook us and rattled the roof of the toll complex," Zarzal said.

Rubalcaba said the damage to the car park was "extensive" and television images showed a number of floors had collapsed on top of each other, with adjoining staircases left twisted and dangling in mid-air.

Rubble had crashed onto numerous cars parked outside the terminal and smoke and dust hung in the air throughout the state of the art building, which only opened in February.

Firefighters had to tackle several fires following the initial explosion. Many parts of the building looked unsafe and about to collapse.

Among the injured, the worst off appeared to be a policeman who received cuts from flying glass.

As recently as Friday Prime Minister Jose Luis Rodriguez Zapatero had insisted he was optimistic that ETA's cease-fire would lead to a definite peace process.

The Spanish government said the attack broke the permanent cease-fire and strongly suggested that the government was withdrawing plans for dialogue with ETA.

"Violence is incompatible with dialogue in a democracy and the government will maintain this come what may," Rubalcaba told a press conference.

"This attack interrupts nine months without violent activity by ETA. It breaks a permanent cease-fire that ETA issued nearly nine months ago," Rubalcaba said.

Zapatero interrupted a family holiday in southern Spain and flew to Madrid to be closer to the events. He was scheduled to make a statement later Saturday.

The bombing seemed sure to be the nail in the coffin of a nascent peace effort championed by Zapatero. ETA has not killed anyone since May 2003, but continued a series of low-level bombings until just before the cease-fire.

Meanwhile, pro-ETA street violence that had halted with the truce resumed in earnest in August, and ETA was blamed for a robbery of 300 pistols in France in October and earlier this month for storing 110 pounds of chemical substances to make explosives found in northern Spain.

More than 800 people have died since the group took up arms in the late 1960s.

The announcement of peace talks was derided by Zapatero's political opponents, who said he was rewarding an active terrorist group whose motives could not be trusted. The Socialist leader also came under fire from ETA and its political supporters, who accused him of dragging his feet on the peace talks and harassing pro-independence militants with police raids and court rulings.

ETA and its outlawed political wing, Batasuna, have also been demanding — and the government refusing — the start of talks among Basque political parties on the region's future. These would be in parallel to talks between the government and ETA on the nuts and bolts of dissolving the armed militancy.

Zapatero said in June he would hold such talks with ETA, but they are not known to have taken place. Pro-ETA street violence that had halted with the truce resumed in earnest in August, and ETA was blamed for a heist of 300 pistols in France in October.

The opposition Popular Party, which has strongly opposed Zapatero's stance, urged the government to break any possible contact with the armed group and to defeat ETA by police and judicial means.

"This attack means that ETA is a criminal organization that doesn't want peace" said Popular Party leader Mariano Rajoy.