Metropolitan Police officers arrested a third man on Tuesday (September 19) in connection with the Parsons Green Tube attack.
The arrest was carried out in Wales by officers from the Met's Counter Terrorism Command at approximately 7.08pm.
A 25-year-old man was arrested under section 41 of the Terrorism Act as investigations into the attack continue.
Detectives made the arrest at an address in Newport, with the support of Gwent Police and the Welsh Extremism and Counter Terrorism Unit (WECTU).
A search is taking place at a Newport address following the arrest.
Met Police also confirmed that two addresses in Surrey and a commercial property in Hounslow are still being searched and this is expected to continue for days.
Two other men were arrested in connection with the attack on Saturday (September 19) and remain in police custody.
An 18-year-old was arrested on Saturday morning by armed police who swooped in on him as he tried to flee the UK at Dover, 24 hours after the attack.
A 21-year-old man was also arrested in Hounslow at around 11.50pm on Saturday (September 16) by detectives from the Met's Counter Terrorism Command investigating the terrorist attack at Parsons Green.
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."
To report any suspicious activity, call Met Police on 0800 789 321, or dial 999 in an emergency.
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