Police in Madrid are investigating the circumstances of a car bomb that blew up Friday night in an underground parking facility. Security experts suspect that the violent Basque separatist group ETA meant to spoil the annual military parade on a national holiday.

A car loaded with an estimated 25 kilograms of explosives blew up at midnight in a car impoundment lot under Madrid's Plaza de Colon. The explosion blew a six-meter crater in the first level of the parking area, injured 17 people, and destroyed several tow trucks and more than 60 cars.

Just 12 hours earlier, the entire Spanish royal family and Prime Minister José María Aznar along with most of his cabinet had sat in a grandstand watching a military parade, just 200 meters from where the bomb went off.

An hour before the parade was due to begin, an anonymous caller speaking in the name of ETA, the Basque separatist group, telephoned to warn that a bomb was set to go off on Alcántara street, just one kilometer from the Plaza. Police immediately combed the area checking license plates and searching with bomb-sniffing dogs; but they found no suspicious vehicles. Seven hours later, police acted on a complaint and removed a white Ford sedan that had been abandoned on a pedestrian crossing on the same street.

The car was towed through the streets of Madrid to the lot where, four hours later, it blew up.

Besides the impoundment lot and parking facilities, the underground complex in the Plaza Colón also houses a station for the airport bus shuttle, a restaurant, and two theaters. A play was being performed when the bomb went off, but spectators and actors only felt the theater shake with a dull thud.

ETA has claimed responsibility for 32 car bomb attacks in Madrid resulting in 55 deaths.