The British police arrested a second man in connection with the bombing on a London Underground train that wounded 30 people, with the United Kingdom home secretary saying the new arrest suggests the attacker was not a "lone wolf".
Scotland Yard's Counter-Terrorism Command investigating the Friday attack claimed by the Islamic State group arrested the 21-year-old from Hounslow in west London on Saturday night.
Earlier, an 18-year-old man was arrested in the port area of Dover. None of the suspects have been named.
Both suspects were being held under the UK's Terrorism Act and being questioned at a south London police station.
Home Secretary Amber Rudd told the BBC that the second arrest suggests the attacker was not "a lone wolf".
"But as this unfolds and as we do our investigations, we will make sure we find out how he was radicalised if we can," she said.
The minister also denied US President Donald Trump had received any leaked security information when he tweeted about the terror attack.
Trump's tweet read: "Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"
Rudd added her voice to British Prime Minister Theresa May's rebuke to Trump on the matter: "It's never helpful to have speculation about an ongoing operation, and I would include the President of the United States in that comment.
"It is pure speculation, absolutely."
She added that 24 million pounds of new government money was going to counter-terrorism operations across the country.
Thirty people were injured during the attack in which an improvised explosive device was detonated on a Tube train at Parsons Green underground station during morning rush hour.
Neil Basu, the Met Police's Senior National Co-ordinator for Counter Terrorism Policing, said the Metropolitan Police and its partners were working to arrest those responsible "for this cowardly crime."
Basu said in a statement his department was "still pursuing numerous lines of enquiry and at a great pace."
Met Police counter-terrorism specialist firearms officers yesterday evacuated buildings as they began searches at a residential address in Sunbury, Surrey, southeast England.
The search remains ongoing as it emerged that the home belongs to an elderly British couple honoured with an Member of the Order of the British Empire medal by Queen Elizabeth II in 2010 for their efforts at fostering hundreds of refugee children.
The 18-year-old arrested is believed to have been one of the children fostered by Ronald Jones, 88, and Penelope Jones, 71.
The UK terror threat level remains 'critical', meaning an attack is expected 'imminently'.
The Islamic State group has said it was behind the bombing but Met Police's Assistant Commissioner Mark Rowley said it was "very routine" for IS to claim the attack, whether in contact with those involved or not.
The explosion, described as a "bucket bomb", sent a "fireball" through the Tube causing burn injuries to several commuters.
The main device, which had been fitted with a crude timer using shop-bought fairy lights, failed to detonate, meaning hundreds of people were spared death and serious injury.
IMAGE: Passengers leave Parson's Green Underground station after it reopened following an explosion on a rush hour train on Friday morning. Photograph: Peter Nicholls/Reuters