The suicide bombing in Manchester last month that killed at least 22 people was being planned since December, BBC reported citing security officials in Libya.
Bomber Salman Abedi, a Briton of Libyan heritage, had been watched in Libya for more than a month before the attack.
Manchester Arena suicide bomber Salman Abedi is seen in this image taken from CCTV on the night he committed the attack. /VCG Photo
Abedi spent a month with his family in Tripoli, capital of Libya, before returning to Britain to carry out the attack.
Libyan officials told BBC that from the time of his arrival in the country he was under surveillance, along with his brother Hashem and father Ramadan.
It is unclear if that information was shared with the UK. Officials in Tripoli have complained about poor security cooperation with the UK.
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