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.
Sir Keir Starmer, front left, and Boris Johnson arrive in Leigh-on-Sea to pay their respects alongwith Speaker Sir Lindsay Hoyle and Priti Patel. Photo: AFP
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 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.
Teachers, members of the public, the NHS and others can refer individuals to a local panel of police, social workers and other experts who decide whether and how to intervene in their lives.
Engagement in the scheme is voluntary and it is not a criminal sanction.
It is thought Ali, who is of Somali heritage, did not spend long in the programme - and he was never a formal "subject of interest" to the Security Service, MI5.
Meanwhile, the UK's home secretary has said politicians cannot be cowed following Friday's fatal attack on MP Sir David Amess.
Priti Patel's comments came after she laid flowers, alongside Prime Minister Boris Johnson and Labour leader Sir Keir Starmer, at the scene of the stabbing in Leigh-on-Sea, Essex.
Sir David, a Conservative MP since 1983, was holding one of his regular Friday meetings with his constituents at Belfairs Methodist Church when he was stabbed multiple times.
The 69-year-old was married with four daughters and a son. He is the second MP to be killed in recent years, following the murder of Labour MP Jo Cox in July 2016.
Speaking at Southend police station, Patel paid tribute to Sir David as a "man of the people" who was killed doing "a job he loved".
Sir David Amess pictured with British paralympian Kadeena Cox Photo: AFP
She said security measures were being put in place to protect MPs - but insisted they would carry on serving the country unimpeded.
"We will carry on, we live in an open society, a democracy," she said.
"We cannot be cowed by any individual or any motivation... to stop us from functioning, to serve our elected democracy."
Patel maintained a balance could be found to allow face-to-face meetings with constituents to continue.
Some MPs held their regular constituency surgeries on Saturday as normal. However, others have raised concern for their safety at sessions where anyone can attend.
Conservative MP Tobias Elwood - who came to the aid of a stabbed police officer during the 2017 terror attack in Westminster - suggested MPs speak to constituents on the phone or over Zoom for the time being, rather than meeting face-to-face.
Labour MP Diane Abbott said she would prefer to meet constituents behind a screen to prevent possible stab attacks.
Kim Leadbeater, the sister of Cox, said her partner had asked her to stand down as MP for Batley and Spen after Sir David's death.
Veteran Labour MP Harriet Harman said she would write to the prime minister to ask him to back a Speaker's Conference - a type of cross-party inquiry - to review parliamentarians' safety.
Local police forces are contacting every MP to discuss their personal safety and the security at events they plan to attend.
The UK's threat level remains at "substantial", meaning a terror attack is likely.
Flags across Parliament are being lowered to half-mast following the killing of MP Sir David Amess at Belfairs Methodist Church in Leigh-on-Sea. Photo: AFP
Tributes for slain MP
Tributes to Sir David have been pouring in from politicians as well as his local constituents.
The prime minister described him as "one of the kindest, nicest, most gentle people in politics", with an "outstanding record of passing laws to help the most vulnerable".
Labour's Sir Keir said it was a "dark and shocking day", adding that the country had "been here before" with the death of Jo Cox.
Father Jeff Woolnough, a parish priest who led a mass on Friday evening in Sir David's memory, described him as a "great, great guy" and said faith communities had "lost their greatest supporter".
Members of Southend-On-Sea's Muslim community said, in a joint statement, his death was an "indefensible atrocity" and that Sir David would be remembered for his warmth, selflessness and kindness.
- BBC