MADRID, April 3: A police officer was killed and eight other people were injured in an explosion in a Madrid suburb on Saturday where police were hunting for suspects in the March 11 bombings, the Web site of newspaper El Mundo said.

An Interior Ministry source earlier confirmed an operation was under way in the suburb of Leganes linked to the investigation, but declined to provide further details.

Shots were fired and an explosion was heard after the deployment of police in large numbers, witnesses cited by national radio said.

Dozens of officers cordoned off part of the suburb of Leganes, asking residents to remain indoors, as a police helicopter circled overhead.

The blast was a controlled explosion set off by security forces, said the police who would not confirm that shots had been fired.

Speaking on Spanish public radio, a witness said that police had tried to arrest three people of north African descent who took shelter in an apartment block. Police were surrounding the building at around 1930 GMT.-Agencies