British police said on Monday they had arrested a 16th person in connection with the Manchester suicide bombing last week.
In addition, police raided a property in a southern Manchester suburb as they step up their The attack, Britain's worst in more than a decade, was carried out by British-born terrorist Salman Abedi, who detonated a powerful bomb at the exit of an Ariane Grande concert.
The source told Reuters that Abedi was not among the 3,000 people now under active investigation by MI5, although he was one of around 20,000 people known to the agency, whose focus is on countering terrorism and espionage.
On Sunday, police arrested a 19-year-old man in Gorton and a 25-year-old man in the Old Trafford area of Manchester on suspicion of terror offenses.
The authorities have released a photo of the bomber Salman Abedi who police believe assembled his device in an appartment in the city centre.
With the massive operation to dismantle his terror network showing little sign of slowing, 14 men were being held in custody in connection with the attack.
The domestic security service is said to be investigating whether any glaring errors were made in the handling of intelligence concerning Abedi before he launched the attack last Monday night.
At least 22 people, including children - one as young as eight-years-old - were killed and 59 injured in a "callous" suicide attack in the northern English city of Manchester on Monday night.More news: FA Cup win won't disguise poor season, says Welbeck
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"The operation is still at full tilt", Home Secretary Amber Rudd said, adding that some suspects could remain at large.
The BBC also said that Abedi had taken part in the armed uprising against Libyan dictator Muammar Gaddafi's regime as a teenager during school holidays.
The attack, Britain's worst in more than a decade, was carried out by British-born extremist Salman Abedi, who detonated a powerful bomb at the exit of a concert by United States singer Ariane Grande.
Investigators say they have a 1,000-strong team working on the probe and have significant details on Abedi's associates and movements, his finances, and how the explosive was built.
Abedi's brother and father are being held in Libya.
The terror threat level now indicates that an attack remains highly likely rather than imminent.
She said the government had "never understated" the threat of a terror attack in the United Kingdom, adding: "We have to level with people".
The family of one victim, 18-year-old Georgina Callander, said her life had been cut short by "evil, evil men prepared to ruin lives and destroy families".