Britain's MI5 has begun an internal review of how it handled intelligence on Manchester suicide bomber Salman Abedi, who was reportedly known to authorities but not under active investigation.
Interior minister Amber Rudd said on Monday the review was the "right first step" for the intelligence agency to take in the wake of the May 22 bombing that killed 22 people at a pop concert by US singer Ariana Grande.
MI5 is subject to scrutiny by a committee of parliament and it is highly unusual for British authorities to make public that the security service is conducting its own investigation into possible lapses.
"The review will look at what was known about Abedi, what decisions were made about the intelligence and what, if anything, could have been done differently," according to a source speaking on condition of anonymity with Reuters news agency.
"This is a review that would seek to answer whether there are lessons to be learned from how the Security Service handled the intelligence on Abedi."
The source said Abedi was not among the 3000 people under active investigation by MI5, although he was one of about 20,000 known to the agency, whose focus is on countering terrorism and espionage.
The BBC said MI5 was alerted at least three times to the 'extremist views' of Abedi, a 22-year-old who grew up in Manchester in a family of immigrants from Libya. It was not possible to confirm that report.
"This is an ongoing investigation so I'm not going to be drawn into comments on the actual man who committed this crime," Rudd told BBC television, declining to say what was known about Abedi and when.
Last week's attack, the deadliest in Britain since 2005, was claimed by Islamic State. It drew particular revulsion because of the targeting of children - the youngest victim was just eight and nine of the others were teenagers.
Earlier on Monday, police made a 16th arrest as part of the case.
Britons head to the polls on June 8 to elect a new government, with security and police cuts having risen to the top of the political agenda since the bombing last Monday.
Prime Minister Theresa May's Conservatives have seen their poll lead cut in the wake of the attack and after a U-turn over their social care plans for the elderly.
Surveys suggest May - who as a former interior minister oversaw the police and domestic intelligence agency - might not win the landslide predicted just a month ago.
It was not clear whether the authorities became aware of Abedi during May's tenure as interior minister between 2010 and 2016.
