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.
Police said a man, who was held on suspicion of the MP’s murder in Essex on Friday, was now being held at London police station after being rearrested under the Terrorism Act.
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