Attack now believed to be less serious than first announced.
Police have made a second arrest, a 16-year-old boy from west London, in connection with the investigation into an alleged data theft from TalkTalk.
He was arrested on suspicion of Computer Misuse Act offences after detectives searched a Feltham address, and later bailed, Scotland Yard said.
On Monday, another boy, 15, was arrested and bailed in Northern Ireland in connection with the hacking.
The TalkTalk website was hit by a “significant” cyber-attack last week.
The phone and broadband provider, which has over four million UK customers, said banking details and personal information could have been accessed.
Officers have also searched a residential address in Liverpool, the Metropolitan Police said.
News of the hacking first emerged last Wednesday, with the telecoms company publicly warning customers that their details could be used by fraudsters.
Chief executive Dido Harding later said the scale of the attack was “smaller” than originally thought, and that any credit card details accessed would have been incomplete.
MPs are to launch an inquiry into the cyber-attack, with culture minister Ed Vaizey saying the government is not against compulsory encryption for firms holding customer data.