LONDON - Police in Britain and Italy questioned 21 suspects as they sought to piece together the networks behind the London bombings, probing for possible links between the two sets of attacks and for connections to any accomplices overseas.

Investigators arrested seven people Sunday at a four-story brick apartment building in Brighton, on England's southern coast, and also searched another home in the city. They gave few details about what role the six men and seven women were suspected of playing in the failed July 21 attacks on the capital's transit system.

So far, 18 people are in custody in Britain and three in Italy.

Police say the four suicide bombers who carried out the July 7 attacks, which killed 52 victims, are all dead. And they believe they have arrested all the failed July 21 bombers, whose explosives detonated only partially and took no lives.

Now they are searching for those who may have recruited and directed the attackers and built the explosives, anxious to catch them before they strike again.

Investigators are also searching for links between the two terror cells, one made up mostly of Pakistani Britons and the other mainly of east African immigrants to London. The groups struck two weeks apart, each attacking three London Underground trains and a red double-decker bus.

In Italy, authorities were pursuing contacts linked to Osman Hussain, 27, who was arrested in Rome on Friday and is suspected of trying to bomb the Shepherd's Bush subway station in west London.

Police have discovered that Hussain called Saudi Arabia hours before his arrest, a newspaper reported. Another newspaper said a bombing suspect, Ethiopian-born Briton Muktar Said Ibrahim, 27, took a monthlong trip to Saudi Arabia in 2003, telling friends he was to undergo training there.

Britain was facing questions about how Hussain slipped out of the country five days after the attempted attacks, despite a massive police manhunt. Italy's Interior Minister Giuseppe Pisanu says Hussain left London's Waterloo station by train for mainland Europe on July 26.