By Paul Sandle
LONDON (Reuters) - British police arrested a second man over the bombing of a London commuter train on Friday that injured 30 people and the security services lowered the threat level for an attack from its highest setting.
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 four miles (six km) 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 Islamic State 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 so-called Islamic State, or Daesh, 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 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.
Local media said the home belongs to a couple who have fostered hundreds of children, including refugees. The BBC said the couple, 88-year-old Ronald Jones and Penelope Jones, 71, had been honored by Queen Elizabeth for their work with children.
The bomb struck as passengers were traveling toward the center 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.
Prime Minister Theresa May put Britain on its highest security level late on Friday and soldiers and armed police were deployed to strategic locations such as nuclear power plants.
On Saturday, armed police patrolled the streets near government departments in Westminster and guarded Premier League soccer grounds hosting matches.
The last time Britain was put on "critical" alert was after a suicide bomber killed 22 people at the Ariana Grande concert.
On that occasion, the threat level remained at critical for four days while police established whether the bomber had worked alone or with others. Prior to that it had not been triggered since 2007.
(Additional reporting by Andrew Heavens; editing by David Clarke)