LONDON – Police arrested seven people yesterday during a raid on an apartment in southern England, bringing to 21 the number in custody in the relentless hunt for accomplices in the failed July 21 transit bombings.

Police were also searching for anyone who may have recruited and directed the attackers and built the explosives.

Police arrested the six men and one woman during a search of two buildings in Brighton, on the southern coast, said a Metropolitan Police spokeswoman, speaking on condition of anonymity because her department does not allow her to give her name. So far, 18 people have been arrested in Britain and three in Italy.

She said police believed there were more people at large who were involved in the July 21 attacks, in which four bombs partly exploded, and the deadly July 7 suicide bombings.

Both attacks targeted three subway trains and a double-decker bus. All the July 7 attackers were believed dead; police have in custody four suspects they believed planted the explosives July 21.

“It’s extremely likely there will be other people [who were] involved in harboring [suspects], financing, and making the devices,” the spokeswoman said.

Key suspects were being interrogated in London in relation to the failed July 21 attempts, police said. 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 Mr. Hussain called Saudi Arabia hours before his arrest, the Telegraph newspaper reported, and the Sunday Times said another bombing suspect – Ethiopian-born Briton Muktar Said Ibrahim, 27 – took a month-long trip to Saudi Arabia in 2003, telling friends he was receiving training there.

Britain was facing questions about how Mr. Hussain, also known as Hamdi Issac, slipped out of the country during a massive police manhunt. Italy’s Interior Minister Giuseppe Pisanu says Hussain left London’s Waterloo station by train on July 26.

The Home Office said immigration officials generally do not check the passports of people leaving the country. However, police had asked that checks be made at many departure points after the attacks, including Waterloo, a Home Office spokesman said on condition of anonymity, according to government policy.

Police had released closed-circuit television images of the four bombing suspects shortly after the attacks, but the picture of Mr. Hussain, whose name was not made public until his arrest, was grainy and his face difficult to see. Police put out a clearer image of him a day after his escape.

Mr. Hussain was arrested Friday in Rome at the apartment of his brother Remzi Issac, who also was detained. Yesterday, Italian police detained a second brother of Mr. Hussain, Fati Issac, for questioning, the Italian news agency ANSA said. Fati Issac was accused of destroying or hiding documents sought by investigators, ANSA said

Britain has requested Mr. Hussain’s extradition, which his court-appointed lawyer, Antonietta Sonnessa, said he is likely to fight.

Mr. Hussain also told investigators his cell was not linked to either Al Qaeda or those who carried out the July 7 suicide attack, Italian news reports said.

His lawyer said the plot was loosely organized, put together at the last minute. “There wasn’t a very clearly defined plan, the whole thing was set the day before, in a meeting with this group of friends,” Ms. Sonnessa said.

In addition to the arrests yesterday, police in Manchester, in northern England, detained a man at a train station under anti-terrorism legislation but said they did not believe he was linked to the London bombings. Inspector Mohammed Sultan gave no further details about the arrest at Stockport station.