British police believe they have captured all four men they were seeking over the July 21 London bombings. After the Friday culmination of an international manhunt, three of the men are in custody in Britain and a fourth was arrested in Rome.

Italian authorities started extradition proceedings on Saturday for Osman Hussain, arrested from an apartment in Rome, while police carried out searches throughout Italy.

Pisanu said Hussain had received help from contacts among Italy’s East African immigrant communities after fleeing London on July 26.

‘‘From the investigations, it has been possible to identify a dense network of individuals belonging to the Eritrean and Ethiopian communities in Italy, believed to have helped him cover his tracks,’’ he said.

Pisanu said Hussain, a naturalised Briton, was born not in Somalia—as the government said on Friday—but in Ethiopia.

Britain has requested Hussain’s extradition, for which a hearing was held on Saturday. His state-appointed defence attorney, Antonietta Sonnessa, said the process was at an extremely early stage.

An Italian police source said investigators were able to trace Hussain by tracking his cell phone, which originally belonged to his brother-in-law, a British resident.

The source added that Hussain’s real name was Isaac Hamdi,but the interior ministry declined to confirm or deny this.

Meanwhile, British police were questioning suspects held in Friday’s raids across London. Two of those arrested have been identified as Ibrahim Muktar Said and Ramzi Mohammed.

British media said another man arrested in London on Friday was thought to be Mohammed’s brother. A police spokesman declined to confirm the man’s identity.

The first prime suspect arrested was Yasin Hassan Omar, seized in a Wednesday raid in Birmingham.

12 people are in British police custody in connection with the attacks, police said on Saturday.