Hopes for an end to Basque separatist violence in Spain vanished today when two car bombs exploded in a Madrid neighborhood, killing an army officer and ending a 19-month lull in such attacks.

Police immediately blamed the bombings on the Basque separatist group ETA, which ended a cease-fire last month after failing to win government concessions. Investigators said the attacks had all the earmarks of ETA.

The first car blew up shortly after 8 a.m. (2 a.m. EST) outside an apartment building in a neighborhood full of military families. About 30 minutes after police and rescue workers arrived, a second car exploded about 400 yards away.

Juan Cotino, head of the national police, said the car bombs were set off by remote control and that the target, Col. Pedro Antonio Blanco Garcia, 47, died. He worked in the economic section of army headquarters in Madrid and was married with two children.

Although there was no immediate claim of responsibility, police reported last month that they had intercepted two ETA-linked cars as they were ferrying 3,300 pounds of explosives to Madrid. The group often waits weeks before claiming responsibility for an attack. The last attack attributed to ETA was the killing of a governing party official on June 25, 1998.

Prime Minister Jose Maria Aznar condemned today's attack and vowed the government would not yield in its fight against the Basque group.

"When ETA announced it was ending the pause in its terrorist activity, I said ETA was making a mistake," Aznar said in a television broadcast. "Now is the time to show ETA the consequences of its error."

As darkness fell, more than 3,000 people gathered in Puerta del Sol plaza in central Madrid to protest the bombings. The rally was supposed to start with a silent vigil, but there were spontaneous outbursts as some in the crowd shouted "ETA are murderers!" and "We want justice!" Protesters carried banners that read, "Basques, yes; ETA, no."

ETA wants to establish a Basque nation in northern Spain and southern France, where the Basques have a separate language and cultural identity. The group's initials stand for the Basque words for "Basque homeland and freedom." In its campaign for independence, ETA has killed nearly 800 people since 1968.