A second man has been arrested in connection with a suspected terror attack on a London Underground train on Friday.
The 21-year-old man was arrested in Hounslow at around 11.50pm on Saturday night, the Metropolitan Police said.
He was arrested under section 41 of the Terrorism Act and taken to a south London police station where he remains in custody.
The news comes after police on Saturday arrested at 18-year-old man in Dover in connection with the rush hour attack.
Home secretary Amber Rudd said the second arrest suggested this was not a “lone wolf” attack.
Speaking on the Andrew Marr Show, she said there was “no evidence” to suggest so-called Islamic State were behind the attack.
She added: “But as this unfolds and as we do our investigations, we will make sure we find out how he was radicalised if we can.”
Later on Sunday she announced that the threat level had been lowered from critical to severe.
The incident, in which 30 people were injured, saw an improvised bomb fail to explode properly at Parsons Green station in south west London.
The station reopened on Saturday morning.
An armed police operation in Sunbury-on-Thames, Surrey, is also continuing.
Officers are searching the house of an elderly couple known for fostering hundreds of children, including refugees.
Ronald Jones, 88, and Penelope Jones, 71 were made MBEs in 2010 for services to children for their foster work.
According to the BBC, the couple are believed to be staying with friends following the police raid.
Nearby properties were evacuated as a “precautionary measure” following the arrest in Dover.