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Manchester bomber Salman Abedi is understood to have been one of 20,000 people known to MI5 but not among the 3,000 under investigation.

Abedi, a Manchester-born Muslim whose parents are from Libya, is claimed to have been reported to authorities at least three times because of his extremist views.

Britain’s spymasters have launched two inquiries into how MI5 missed the danger posed by the Manchester bomber.

A source said: "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."

Britain’s domestic security service started one review last week, which will aim to quickly identify any glaring errors, while a second will be more in-depth.

(Image: mi5.gov.uk)

Yesterday the Home Secretary refused to comment on whether opportunities were missed to spot the murderous intent of the 22-year-old before his deadly attack, as national security became the major issue in the general election campaign.

Amber Rudd also warned that terrorists who helped Abedi could still be at large.

She told BBC1's Andrew Marr Show: "It's an ongoing operation, there are 11 people in custody, the operation is still really at full tilt, in a way.

"So until the operation is complete, we can't be entirely sure that it is closed."

The Home Secretary's comments came despite the terror threat level being reduced from the highest level, critical, to severe yesterday.

Police have made a string of arrests amid fears Abedi had help to plan or build his sophisticated shrapnel bomb.

Yesterday cops issued CCTV stills of Abedi, bespectacled and casually clothed, in a plea for information about his movements between May 18 and the attack.

A matter of hours after he was captured on camera, Albedi was dead, having inflicted an outrage on a pop concert attended predominantly by young girls.

The huge police operation that followed saw raids in several cities as counter-terror efforts were focused on cornering his suspected criminal ring.

The Home Secretary said Islamic State, also known as Daesh , was trying to "weaponise" young Britons and claimed a "whole new terrorism industry" is trying to recruit British suicide bombers, rather than fighters in Syria.

"They’ve changed their message from 'Come and join the Caliphate' to 'You can do your damage in your own country'," Ms Rudd told the Mail on Sunday.

Murderer Abedi had been a former "subject of interest" to MI5, but Ms Rudd told the BBC: "The intelligence services are still collecting information about him and about the people around him.

"But I would not rush to conclusions, as you seem to be, that they have somehow missed something."

She added: "What this reminds us is the scale of the problem that we have, the enemy that we have, Daesh, that is trying to weaponise the young people in our society."

The comments came after Ms Rudd launched an extraordinary political attack on Jeremy Corbyn, claiming the Labour leader would "absolutely" mean a higher terror threat if he becomes Prime Minister in the general election.

She told the Mail on Sunday: "The whole [Labour] team, and in particular Mr Corbyn, have boasted about opposing them [security measures].

"I spend two hours every day signing security warrants. The only thing Corbyn would sign is our security away. He’d be a disaster."

She claimed the prospect of Diane Abbott as Home Secretary was "ghastly" saying she was "against the basic tenets of security".

Ms Abbott insisted she could be trusted with tracking terror suspects if Labour wins the general election on June 8.

She said: "I was a Home Office civil servant. I know how these things work.

(Image: PA)

"If the files are put in front of me, evidence is put in front of me, of course I'll sign orders for surveillance."

The interview came as Manchester prepares to close the first week since the terror bombing by Salman Abedi that killed 22 concert-goers at an Ariana Grande gig.

Labour restated its commitment to hire 10,000 more police officers, 3,000 more firefighters, 3,000 more prison officers and 500 more border guards.

Jeremy Corbyn said: "Ensuring the safety of our communities demands properly resourced action across many fronts.

"It means upholding and enforcing our individual rights, promoting community relations, supporting our emergency services, tackling and preventing crime and protecting us from danger, including threats of terror and violence."

Meanwhile the Tories announced a new Commission for Countering Extremism would have a remit to clamp down on "unacceptable cultural norms" such as female genital mutilation, and promote "British values".

It will also act to ensure that women's rights are upheld in all of Britain's ethnic and religious communities, Theresa May said.

Mrs May wrote in the Sun on Sunday: "Enough is enough. We need to be stronger and more resolute in standing up to those who hate our values and want to destroy the freedoms we hold dear".