A man detained at the scene of Friday's fatal stabbing of MP Sir David Amess is 25-year-old Ali Harbi Ali, Whitehall officials have confirmed to the BBC.
Police said he was being held at a London police station under the Terrorism Act 2000 and officers have until 22 October to question him.
They spent Saturday searching three addresses in the London area.
The BBC understands Mr Ali was referred to the counter-terrorist Prevent scheme a few years ago.
Prevent is the UK's terrorism-prevention programme, which aims to stop people being radicalised.
https://www.bbc.co.uk/news/uk-58943184