Salman Abedi was being watched in Libya more than a month before the attack.
Officials in Tripoli have complained about poor security co-operation with the UK, which they say must be improved to prevent further attacks.
Abedi spent a quiet month with his family in Tripoli before returning to Britain to carry out mass murder.
Libyan officials have told the BBC's Orla Guerin that from the time of his arrival in the country he was under surveillance, along with his brother Hashem and father Ramadan.
