British police have arrested a second man over the bombing of a London commuter train that injured 30 people as security services lowered the threat level for an attack from its highest setting.
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The 21-year-old man was detained under Britain's terrorism laws in the west London suburb of Hounslow just before midnight on Saturday, London police said in a statement.
Police earlier arrested an 18-year-old man in the departure lounge of Dover port in what they called a "significant" step and then raided a property in Sunbury-on-Thames, a town near London and about 6km from Hounslow.
The home-made bomb shot flames through a packed carriage at west London's Parsons Green Tube station during the Friday morning rush hour but apparently failed to detonate fully.
Islamic State claimed responsibility, as it has for other attacks in Britain this year, including two in London and one at a concert by American singer Ariana Grande in Manchester in May.
Interior minister Amber Rudd said on Sunday the second arrest showed it was not a lone-wolf attack but there was no evidence IS was involved.
She said the threat level had been lowered to "severe" from "critical", meaning another attack was highly likely rather than expected imminently.
Police said on Sunday they were searching a home in Stanwell, in the county of Surrey near the perimeter of London's Heathrow Airport, in connection with the Hounslow arrest.
Police continued to search the house in Sunbury nearby but said there were no safety risks to local residents.
The bomb struck as passengers were travelling towards the centre of the British capital. Some suffered burns and others were hurt in a stampede to escape. Health officials said none was thought to be in a serious condition.
On Saturday, armed police patrolled the streets near government departments in Westminster and guarded Premier League soccer grounds hosting matches.