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”.
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’.
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 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.”
Source: BBC