Share Media caption Amber Rudd: Second arrests suggests 'not a lone wolf' 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. Home Secretary Amber Rudd told the BBC that the second arrest suggests the attacker was not "a lone wolf". Speaking to the Andrew Marr Show, Ms 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. The UK terror threat level remains 'critical', meaning an attack is expected 'imminently'. Image copy