Manchester bomber Salman Abedi 'phoned mother to say forgive me hours before attack'
Manchester bomber Salman Abedi was known to the security services and his risk to the public remained "subject to review" before he carried out his deadly attack.
Abedi, whose sister said he "wanted revenge" for Western military strikes in the Middle East, was a "former subject of interest" to MI5, a Whitehall source confirmed.
Hours before the attack, he had phoned his mother and said "forgive me", according to a Libyan anti-terror official.
Libyan investigators believe Abedi acted alone when perpetrating the outrage, Special Deterrent Force spokesman Ahmed bin Salem added.
Relatives of the British-born bomber are being interrogated by Libyan authorities in Tripoli, where his father Ramadan and teenage brother Hashim were arrested on Wednesday.
Abedi's mother, Samia Tabbal, was said to have told them her son left the North African country bound for the UK four days before detonating his device at Manchester Arena.
Mr Bin Salem said of the final phone call: "He was giving farewell."
Hashim, 18, allegedly claimed his older brother learnt to build bombs from the internet, hoping to "seek victory for the Islamic State", Mr Bin Salem added.
Despite the belief of the Libyan authorities that Abedi acted alone, British investigative efforts remained focused on smashing the potential terror ring which may have assisted him.
Police hunting the "network" behind his attack said they had made "significant" arrests and seized "very important" items in raids linked to the investigation.
After chairing a meeting of the Cobra emergency committee, Theresa May said the terror threat level will remain at critical - meaning another attack is expected imminently.
In an indication of the level of counter-terrorism activity, a senior Whitehall source revealed that 18 plots had been foiled since 2013 in Britain, including five in just nine weeks since the Westminster attack in March this year.
It is understood the scale of the threat being dealt with by counter-terror agencies is "unprecedented" and intelligence officers faced "difficult professional judgments" about where to focus their investigations.
The source said: "MI5 is managing around 500 active investigations, involving some 3,000 subjects of interest (SOIs) at any one time.
"Abedi was one of a larger pool of former SOIs whose risk remained subject to review by MI5 and its partners.
"Where former SOIs show sufficient risk of re-engaging in terrorism, MI5 can consider reopening the investigation, but this process inevitably relies on difficult professional judgments based on partial information."
Abedi targeted music fans at an Ariana Grande concert at Manchester Arena on Monday night, killing 22 people, including seven children, and injuring dozens in the worst terrorist incident to hit Britain since the July 7 attacks in London in 2005.
As a huge inquiry into the atrocity continued:
:: Eight people remained in custody in connection with the investigation.
:: Officers carried out searches at properties in Manchester, Wigan and Nuneaton.
:: Greater Manchester Police Chief Constable Ian Hopkins said: "I want to reassure people that the arrests that we have made are significant, and initial searches of premises have revealed items that we believe are very important to the investigation."
:: The fallout continued in the extraordinary row sparked by the appearance of sensitive information from the probe in American media, with US President Donald Trump vowing to get to the bottom of "deeply troubling" alleged leaks.
:: The Queen visited some of the youngsters injured in the blast at Royal Manchester Children's Hospital and condemned the "very wicked" attack.
:: NHS England said 23 people remained in critical care across eight hospitals. They include five children at the Royal Manchester Children's Hospital.
:: The nation fell silent at 11am to mark the tragedy.
:: British Transport Police announced that specialist firearms officers were to patrol on board trains nationwide for the first time.
:: NHS England warned health organisations to "ensure care is in place should it be needed" as Britain remained on high alert in the run-up to the bank holiday weekend.
Investigations into Abedi's path to violent extremism were continuing apace.
The British-born son of Libyan parents, he had been banned from a mosque in Manchester after criticising an imam for "talking bollocks" during a sermon critical of the Islamic State (IS) terror group.
There have also been claims that authorities had been warned about concerns he was developing radical views.
The 22-year-old's father Ramadan and brother Hashim have been detained in Libya and another brother, Ismail, was arrested in Manchester on Tuesday.
The killer's sister, Jomana Abedi, told the Wall Street Journal her brother may have been reacting to US-led strikes in the Middle East.
"I think he saw children - Muslim children - dying everywhere, and wanted revenge," she said.
"He saw the explosives America drops on children in Syria, and he wanted revenge. Whether he got that is between him and God."
Meanwhile, apparent leaks of material relating to the investigation, including evidence photographs from the scene of the attack - were said to have caused "distress and upset" to victims' families.
Defending its decision to publish the pictures, the New York Times said: "The images and information presented were neither graphic nor disrespectful of victims, and consistent with the common line of reporting on weapons used in horrific crimes."
The Prime Minister said she would raise the leaks with Mr Trump in the margins of a Nato summit in Brussels, stressing that the "special relationship" was based on trust.
President Trump vowed to investigate, calling the leaks "deeply troubling".