Police have arrested a third man in Wales in relation to the Parsons Green tube attack last week which left 30 injured.
The 25-year-old man was arrested at an address in Newport under the Terrorism Act.
Police are searching a house at the same address.
Two other men were arrested over the weekend after the attack on Friday.
An 18-year-old man was detained on Saturday in Dover, where he was reportedly buying a ferry ticket to Calais. A 21-year-old man was also detained outside the chicken shop where he worked in Hounslow later that evening, as an undercover police officer posing as a homeless person tackled him to the ground.
Searches are also being carried out at two addresses in Surrey and a commercial property in Hounslow. One is where the two men lived with foster parents in Sunbury-on-Thames.
Commander Dean Haydon, head of the Met Counter Terrorism Command, said: “This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday. We now have three men in custody and searches are continuing at four 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. However it is important that we continue with these searches and I'd like to thank all those affected for their support, patience and cooperation.“
Police refused to comment on CCTV footage taken of the 18-year-old suspect walking towards Parsons Green tube carrying a Lidl bag, which contained an explosive cocktail of chemicals in a plastic bucket.
The device sent a fireball down the busy tube carriage, injuring 30 people and causing a stampede at the tube station, but it failed to fully detonate.
The 18-year-old and 21-year-old were Iraqi refugees who came to the UK as teenagers.
