British police make second arrest over London train attack

British police said Sunday they had made a second arrest over the bombing of a London Underground train, as their probe into the terror attack widened.

The 21-year-old man, who has not been named, was detained late Saturday in Hounslow, on the western rim of the capital, a police statement said.

Officers had earlier arrested an 18-yearold man over Friday's attack at Parsons Green station, which injured 30 people, and said they were hunting for more suspects.

Home Secretary Amber Rudd said Sunday that police are trying to find out how the teenager was "radicalized".

The bomb went off in a packed carriage and although the device is thought to have malfunctioned it still caused a large explosion followed by what witnesses said was a "fireball".

It was Britain's fifth terror attack in six months -- a series that has claimed 35 lives.

The Islamic State group claimed responsibility for Friday's explosion.

The first arrest on Saturday took place at the Dover ferry terminal -- a main link to Europe. A "number of items" were recovered during the operation and the teenager is now in custody in London, officers said.

Police also raided a home in Sunbury, a town west of London on Saturday. Local residents quoted in British media said the owners of the house were elderly foster parents.

Britain's terror threat was raised on Friday to "critical", indicating that another attack is feared, and soldiers have been deployed to guard key sites, including nuclear facilities.

The critical warning was last used after the deadly suicide bombing at a pop concert in the northwestern city of Manchester in May that was also claimed by IS.

Rudd voiced doubt over the IS claim it was behind Friday's incident.