The bomb attack in Manchester last month which killed 22 people was being planned since December, security officials in Libya have told the BBC.
Officials in Tripoli have complained about poor security co-operation with the UK, which they say must be improved to prevent further attacks.
Salman Abedi spent a quiet month with his family in Tripoli before returning to Britain to carry out mass murder. Abedi was being watched in Libya more than a month before the attack.
It is unclear if that information was shared with the UK, our correspondent says.
Hashem Abedi and Ramadan Abedi are still being interrogated by Libya’s Special Deterrence Force. Read more at BBC.