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.
The terror threat level has now been lowered to “severe” from “critical”, Home Secretary Amber Rudd has said.
This means an attack is highly likely, and follows the terror threat level temporarily being raised to critical, meaning an attack may be imminent, on Friday evening.
Police are searching a residential address in Stanwell, Surrey, in connection with the 21-year-old man’s arrest.
Rudd said that over the next few days the military would return to their original positions.
She said the police had made “good progress” in the investigation but urged “everybody to continue to be vigilant but not alarmed”.
Assistant Metropolitan Police Commissioner Mark Rowley said police had gained a “greater understanding” of how the bomb was prepared but said there was “still much more to do” in the investigation.
Speaking to the Andrew Marr Show earlier, 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.
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. (BBC)