Police stepped up their investigation Monday into the bombing of a packed London Underground train as commuters headed to work in the first morning rush hour since the attack last week.
The bomb went off on Friday morning in a crowded carriage at the Parsons Green Underground station. Although the device is thought to have malfunctioned, it still wounded 30 people.
Britain downgraded on Sunday the nation's terrorism threat from its highest level following the arrest of a second suspect.
Interior Minister Amber Rudd said the second arrest showed it was not a lone-wolf attack but there was no evidence ISIS was involved.
She said the threat level had been lowered to "severe" from "critical", meaning another attack was highly likely rather than expected imminently.
It is inevitable that ISIS “will reach in and try to claim responsibility. We have no evidence to suggest that yet," Rudd told the BBC.
"But as this unfolds, and as the police do their investigations, we will make sure that we find out exactly how he was radicalized, if we can."
Police said earlier Sunday that a 21-year-old man, who has not been identified, was detained late Saturday in Hounslow, on the western rim of the capital.
A search was underway on Sunday in Stanwell, a few miles west of Hounslow, in connection with the man’s arrest, police said.
After taking into custody an 18-year-old suspect earlier on Saturday over the "bucket bomb" attack, police said they they were hunting for more suspects.
Police haven't charged the two men. Both are being questioned in a south London police station.
Meanwhile, closed-circuit television images, acquired by ITV News and broadcast Sunday, appeared to show a person walking with a shopping bag in the suburb of Sunbury before Friday’s attack.
Images posted on social media following the attack appeared to show wires protruding from a flaming bucket contained in a bag on the floor of the train carriage.
The bomb only partially exploded. Officials say the injuries would have been far worse if it had fully detonated.