Two more persons were on Wednesday detained by the British police as part of their investigation into the London Tube "bucket bomb" attack in which 30 people were injured, bringing the total number of arrests to five.
A 48-year-old man and a 30-year-old man were detained by Scotland Yard's Counter-Terrorism Command under the UK's Terrorism Act this morning, after a search at an address in Newport, South Wales, the Metropolitan Police said.
"This continues to be a fast-moving investigation. We now have five men in custody and searches are continuing at four addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack," said Commander Dean Haydon, head of the Met Police Counter-Terrorism Command.
"We are asking the public to look out for anything that seems out of place, unusual or just doesn't seem to fit in with day-to-day life," he added.
Wednesday's arrests in Newport follow the arrest of a 25-year-old in the same Welsh town.
He remains in custody after the operation led by the Met's Counter Terrorism Command and supported by colleagues from Gwent Police and the Welsh Extremism and Counter Terrorism Unit.
On Saturday, an 18-year-old man was detained at Dover port, and a 21-year-old was arrested in Hounslow, west London.
Both are still being questioned in custody, police said.
The suspects have not been named officially until they go on to be formally charged but the 21-year-old has been identified locally as Syrian refugee Yahyah Faroukh.
All arrests have been made under Section 41 of the UK's Terrorism Act.
A homemade "bucket bomb" partially exploded in a train at Parsons Green station in west London, injuring 30 people during rush hour on September 15.
Relatives of Farroukh, who remains in custody, have spoken of their shock and disbelief at his arrest.
Fatin Farroukh, his sister-in-law, said he was a well-adjusted, hard-working young man with hopes of going to university in the UK and becoming a journalist.
"We have many relatives in Syria who are named Yahyah; we thought it was one of them who was arrested in Damascus. We never thought of Yahyah in London -- he was the last person that we would think would be arrested," she said from her home near Breda in the Netherlands.
The UK police have been given until tomorrow's time to continue questioning Farroukh and until Saturday to continue questioning the unnamed 18-year-old suspect, who like Yahyah Farroukh had been in the care of foster parents Pauline and Ronald Jones at their home in Sunbury-on-Thames in Surrey, south-east England.
The home is now believed to be the site where the terror suspect responsible for the September 15 attack put together his home-made bomb, fitted with a crude timer using shop-bought fairy lights.
The CCTV images have since emerged of a young man carrying a supermarket bag in the street next to the Joneses’ home.
Ninety minutes later, a bomb that had been constructed inside a plastic bucket and placed inside a similar bag partly detonated on board the stationary Tube train in west London.
Experts believe that the initiating charge of the improvised explosive device had exploded but had failed to detonate the main charge.
Following the first arrest last weekend, the UK government had downgraded the country's terror threat level from critical -- meaning an attack was imminent -- to severe, signalling an attack remained likely.
Representative image. Photograph: Hannah McKay/Reuters