A United Kingdom security official told The Wall Street Journal Monday that suicide bomber Salman Abedi, who massacred 22 people at an Ariana Grande concert in Manchester on May 22 via a bomb in his backpack full of metal screws, was known to MI5 but not under active investigation.
"The release of some people can be expected in investigations of this nature as we corroborate accounts that have been provided", said Russ Jackson, head of the north-west counter-terrorism unit.
The investigation brought the arrest of a 23-year-old man in Sussex, south of London, on suspicion of terrorism offenses.
Britain's domestic security service was alerted to Abedi's extremist views on at least three occasions before last Monday's terror attack, which left 22 people dead, but decided against investigating further.
Greater Manchester Police said two men aged 20 and 24 from the Fallowfield area and a 37-year-old man from the Blackley area had been released.
British police also apprehended a 19-year-old man during a raid of the former home of the Salman Abedi, the bombing suspect.
Abedi could be seen wearing jeans and trainers, a black bodywarmer and a baseball cap, with the straps of the backpack believed to contain the bomb visible on his shoulders.More news: May, Corbyn set out EU no deal stances
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"I want to stress that this is a different item than the one he used in the attack", Superintendent Jackson said.
Manchester City Council has called a vigil in the centre of the northwest England city for 22.31pm - the exact moment that Abedi detonated his bomb outside a pop concert by teen idol Ariana Grande in one of Europe's biggest indoor arenas.
The CCTV still picture was taken in Manchester city centre on the day of the attack last Monday. Where did you see him with it during that time?
Britain's Security Service MI5 has launched two urgent investigations in order to understand how the security agencies within the country missed the warnings indicating that the Manchester Arena terrorist was planning an attack.
Abedi's brother and father are being held in Libya.
A total of 16 people have been arrested in connection with the attack, in which 22 people died.
Police are scouring 12,960 hours of closed circuit TV recordings in their search for clues, have collected more than 630 pieces of evidence and have searched 21 addresses.
British officials have confirmed that he had recently returned from Libya and the officers said police needed information about his movements from May 18 when he returned to Britain.