The man arrested by police following the killing of the MP Sir David Amess has been named as Ali Harbi Ali.
The 25-year-old is being held under the Terrorism Act and officers have until Friday to question him.
The BBC understands Mr Ali was referred to the counter-terrorist Prevent scheme some years ago, but was never a formal subject of interest to MI5.
Whitehall officials told the BBC that the man being held was Ali Harbi Ali, a British man of Somali heritage.
Early investigations revealed a potential motivation linked to Islamic extremism, police said on Friday.
Officials do not believe anyone else was involved in the attack.
Sir David, who had been a Conservative MP since 1983 and represented the Southend West constituency, was stabbed multiple times as he held a regular Friday meeting with constituents at Belfairs Methodist Church in Leigh-on-Sea