British police detained on Wednesday two more men over the bombing of a packed London Underground train last week which injured 30 people.
The men, aged 48 and 30, were held under terrorism laws in Newport in south Wales. Another 25-year-old man was arrested on Tuesday evening, London’s Metropolitan Police said in a statement.
Two other men who have been detained in connection with the attack at Parsons Green underground station remain in police custody.
"This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday," said Dean Haydon, head of the Metropolitan Police's Counter Terrorism Command.
"Detectives are carrying out extensive inquiries to determine the full facts behind the attack."
The home-made bomb went off on Friday during the morning rush hour on the packed train, engulfing the carriage in flames.
On Saturday, police arrested an 18-year-old man in Dover, southeast England, and a 21-year-old in west London over the attack. Neither has been charged.
The improvised device on the underground train appeared to have been prepared in a white bucket which had been placed inside a frozen food bag. The blast inflicted flash burns on passengers when it went off.
Following the attack Britain raised its terrorism threat level to the maximum, but downgraded it from critical to severe on Sunday.
A total of 36 people have been killed in Britain this year in militant attacks.