Two loud booms rattled windows and frightened residents in the southern city of Malaga today, but Spain's Interior Ministry said the noise was caused by a plane breaking the sound barrier, and police reported no incidents anywhere in the city.
The thunderous sounds scared residents fearful there might have been a terror attack, particularly in the wake of stepped up violence by the Basque separatist group ETA, which on Monday killed an army officer in an attack in the north of the country.
But police found no reports of any incident, and a few minutes later, officials said they had pinpointed the cause of the booms: a military plane breaking the sound barrier.
"Be calm," a spokesman for the Interior Ministry advised residents in an interview with The Associated Press. "A plane that was flying lower than normal has broken the sound barrier and made this huge sound."
Conchita Vila, the owner of the El Tubo bar in Malaga, said the blast had shaken her apartment.
"It scared me, man," she told the AP in a phone interview. "I live on the first floor and the whole house moved."
The loud sounds — which occurred at about 9.40am. local time — caused a flood of frightened calls to authorities, and prompted several media reports that explosions had gone off, adding to the sense of unease.
It was not clear why the plane was flying so low over a populated area, or where it had come from. There are several Spanish military bases within a few minutes flight of Malaga, as well as an American air force refueling base in the town of Moron de la Frontera.
Sonic booms are caused when an aircraft exceeds the speed of sound — about 750 mph (1,200kph) at sea level — when air pressure waves merge to form shock waves. Those are heard as a sonic boom when they reach the ground.