Libya is working closely with Britain to identify possible "terrorist networks" involved in the attack at a pop concert in Manchester, a senior Libyan official said Thursday.
A source close to the family of the suspected suicide bomber Salman Abedi, a Manchester-born Briton of Libyan origin, meanwhile, said he had expressed a desire to avenge the killing of a friend in the English city last year.
"We are working closely and intensively with our British partners to probe possible terrorist networks and we have achieved important progress," deputy interior minister Abdelsalam Ashour told reporters.
Ashour, a member of the UN-backed Government of National Accord (GNA), said the interior ministry's Deterrence Force was in charge of the investigation.
"We have strong and close ties with our (British) friends," he told reporters, reading from a statement.
The official said GNA head Fayez al-Sarraj has spoken on the phone with Britain's Prime Minister Theresa May and pledged Libya's "full coordination".
Libyan authorities have detained the alleged suicide bomber's father as well as a brother, sources in Libya have said.
The father, Ramadan, was once a member of a Libyan militant group with alleged ties to Al-Qaeda, according to a Libyan security source.
Ahmed bin Salem, spokesman for Libya's Deterrence Force, which acts as the police for the unity government, said belonged to the Libyan Islamic Fighting Group (LIFG).
Ramadan Abedi was hunted by Moamer Kadhafi's regime for his ties to the group, finding refuge in Britain before returning to Libya in 2011 to join the NATO-backed uprising that finally overthrew the dictator, British media have reported.
In Britain, investigators are trying to track down a jihadist network suspected of having orchestrated the attack on Monday night at a pop concert that killed 22 people, many of them children, and wounded dozens more.
After arresting a 23-year-old man on Tuesday, British police said they had taken three more men into custody on Wednesday in south Manchester, where Abedi lived.
The attack was claimed by the Islamic State group.
The source close to Abedi's family said he wanted to avenge the killing of a friend, also of Libyan descent, who had died after being stabbed by British youths in Manchester in May 2016.
"That incident stirred up a sense of anger among young Libyans in Manchester and especially Salman, who clearly expressed his desire for revenge," said the source, declining to be named.
"We were able to calm the young people in the neighbourhoods who felt they were targeted... as Muslims," he said. "But it seems that Salman did not forget the incident."
"I personally talked with him and tried to convince him that it was just a criminal act," he added.
British media reported that Abdul Wahab Hafidah died after being run over and stabbed in the neck in Manchester's Moss Side district in May last year.
His suspected killers are still on trial.