The bomb attack in Manchester last month which killed 22 people was being planned since December, security officials in Libya have told the BBC.
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.
It is unclear if that information was shared with the UK, our correspondent says.
Hashem Abedi and Ramadan Abedi are still being interrogated – daily – by Libya’s Special Deterrence Force. Its spokesman Ahmed Ben Salem told the BBC the attack was being planned as far back as last December.
He said the force had important information about Abedi’s contacts in the UK and Libya.
But security officials in Libya say they have closer co-operation with the CIA than with London.
A general with the UN-backed government in Tripoli said they wanted better intelligence sharing – as quickly as possible – to avoid another Manchester.
Abedi’s brother Hashem has admitted while in detention that they both joined the group known as Islamic State (IS) in 2015, a Libyan security official said.
It is not clear if the information was given freely or under duress.
Libyan officials say Hashem Abedi bought parts for the bomb while in the UK. He left the UK on the same day as his brother.
Police in the UK meanwhile have said that Abedi working alone in his flat was likely to have built the device that killed 22 people.
Officers said there was now a “deep understanding” of the terrorist’s movements in the weeks leading up to the attack at the Manchester Arena on 22 May.
The head of counter-terrorism said it was “less clear” whether he solely obtained and stored all the materials or if others were “complicit”.
Early in the investigation, police said they could be looking for a “network” of people in connection with the attack, but most of those arrested have now been released.