A second man has been arrested in connection with Friday's attack on a London Tube train, police said.
The 21-year-old man was arrested in Hounslow, west London, on Saturday night on suspicion of a terror offence and is in custody in south London.
An 18-year-old man is also being held on suspicion of a terror offence over the Parsons Green explosion.
Home Secretary Amber Rudd told the BBC that the second arrest suggests the attacker was not "a lone wolf".
Police are searching a residential address in Stanwell, Surrey, in connection with the 21-year-old man's arrest.
Speaking to the Andrew Marr Show, Ms Rudd said there was "no evidence" to suggest so-called Islamic State were behind the attack.
"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 UK terror threat level remains 'critical', meaning an attack is expected 'imminently'.
Of the 30 people injured in Friday's attack, one remains in hospital, NHS England said.
Police are continuing to search a house in Sunbury-on-Thames in Surrey.
It is thought the 18-year old, who was arrested in the port of Dover on Saturday morning, lives there.
The BBC's home affairs correspondent Dominic Casciani, who is at the scene, said a large number of officers have been working inside the property and a forensics tent is in place in the back garden.
The house belongs to an elderly couple known for fostering hundreds of children, including refugees.
Ronald Jones, 88, and Penelope Jones, 71 were rewarded for their service to children when they were made MBEs in 2010.
The couple are said to be staying with friends following the police raid, during which surrounding houses were evacuated.
Friend Alison Griffiths said the couple had an 18-year-old and a 22-year-old staying with them recently.
She described Mr and Mrs Jones as "great pillars of the community", adding: "They do a job that not many people do."
Shadow Home Secretary Diane Abbott raised concerns about police resources amid a rise in terrorist attacks.
She told BBC Breakfast that cuts in policing have "possibly jeopardised our ability to respond to terrorism".
But Ms Rudd said the government is "committed to making sure the security services and the police have the right resources".
She announced £24m of funding for counter-terrorism operations across the country.
She added that US President Donald Trump's suggestion that the terrorist behind the attack was "in the sights of Scotland Yard" was "pure speculation".
The police urge anyone with information to get in touch and to upload pictures and video to the website www.ukpoliceimageappeal.co.uk or to call the Anti-Terrorist Hotline on 0800 789 321.
Did you witness the arrest in Hounslow? Share your pictures, video and experiences by emailing firstname.lastname@example.org.