The man held after the fatal stabbing of Tory MP Sir David Amess had been referred to the Government’s counter-extremism programme.
The 25-year-old – named as Ali Harbi Ali – is reported to have been reported to Prevent several years ago.
It is not revealed if he was known to MI5.
The suspect’s father is said to have been an official in the Somalian government.
The police have been granted a warrant of further detention to continue to question the man arrested at the scene.
He can be held in custody for another week.
Sir David, 69, died after being stabbed “multiple times” at his constituency surgery.
The married father of five was the MP for Southend West in Essex.
The incident happened at the Belfairs Methodist Church in Leigh-on-Sea.
It is reported Sir David was initially treated at the scene by paramedics but later died from his injuries.
