A 17-year-old boy has been arrested in south London over the Parsons Green terror attack.
Police in the UK said he was arrested in Thornton Heath just after midnight on Thursday after officers carried out a search warrant.
He was detained under the UK's Terrorism Act.
It means six people are now being held in connection with Friday's attack, which saw a crude home-made bomb left on a London Underground train.
No-one was killed when the device failed to properly detonate, but 30 passengers were injured.
Thursday's arrest follows those of three men in Newport on Tuesday night and Wednesday morning. Their ages are 25, 30 and 48.
The latter two were said to have been arrested in a "halfway house" for asylum seekers, according to neighbours.
Police are searching properties in the Gwent town, two addresses in Surrey, and have started searching the Thornton Heath property.
An 18-year-old, arrested in Dover on Saturday, and suspected of planting the device, is also still in custody after police got a warrant to hold him until 23 September.
The sixth man, Yahyah Farroukh, 21, was arrested at a fried chicken shop in Hounslow, west London, on the same day.
A warrant for his detention is expected to expire today.
Both Farroukh and the 18-year-old are believed to have spent time in the care of Penelope and Ronald Jones, from Sunbury-on-Thames, who received MBEs for services to children and families in 2010.
The Joneses have been highly respected foster parents for almost 40 years and looked after up to 300 children, including eight refugees.
Commander Dean Haydon, head of the Met Police's Counter Terrorism Command, said: "We now have six males in custody and searches are continuing at five addresses.
"Detectives are carrying out extensive inquiries to determine the full facts behind the attack.
"We anticipate that the searches will take some days to complete and may cause further disruption."