London (CNN)British police have arrested a 17-year-old in connection with last week's attack on the London Underground at Parsons Green station, bringing the number held to six.
Parsons Green tube bombing: Police make sixth arrest as inquiry continues
The teen was taken into custody after detectives executed a warrant at an address in Thornton Heath, south London, early on Thursday.
Search operations were continung on Thursday at five properties, including the address where the teen was arrested overnight.
An improvised bomb detonated on a busy commuter train last Friday during the morning rush hour as as it pulled into Parsons Green, injuring 30 people. It appeared only to have partially detonated.
"This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday. We now have six men in custody and searches are continuing at five addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack," said Cmdr. Dean Haydon, head of the Met's Counter Terrorism Command, who said the searches could go on for "some days."
The six suspects are being detained at a south London police station.
Three men aged between 25 and 48 were arrested in the southern Welsh city of Newport by the Metropolitan Police Counter Terrorism Command in connection with local law-enforcement officials earlier this week.
The remaining two men were arrested the day after the attack: An 18-year-old was detained in the port of Dover as he prepared to board a ferry service to France, and a 21-year-old was arrested in Hounslow, west London.
Police have not released the names of either of those suspects but both are believed to have been fostered by an elderly couple, Ronald and Penelope Jones, local officials told CNN on Monday.
Alison Griffiths, a Surrey County councillor, described the Joneses as "the sort of people who would never turn a child away, no matter what the circumstances."
Griffiths said the Joneses had fostered more than 280 children over the past 30 years.
They have fostered about eight refugees in the past year, including children from Afghanistan and Iraq, she added.
"This has come as a big shock to them. You don't expect it in your own house."
The couple were given medals by Queen Elizabeth II in 2010 for the continued work fostering children and young people.
ISIS has claimed involvement in the subway bombing via its Amaq News Agency.
But UK Home Secretary Amber Rudd said on Sunday afternoon: "It is inevitable that so-called Islamic State or Daesh will reach in and try and claim responsibility. We have no evidence to suggest that yet."