The terror network behind Manchester bomber Salman Abedi could still be at large, the Home Secretary said, as the hunt for accomplices continued at "full tilt".
Amber Rudd said members of the suicide attacker's circle were "potentially" unaccounted for, despite optimism that a wave of arrests had quelled further threats.
Teenage couple Chloe Rutherford, 17, and Liam Curry, 19, from South Shields, and Jane Tweddle, originally from Hartlepool, were among Abedi's 22 murder victims on Monday night.
As the fallout from Monday's atrocity continued to be felt, the family of one victim warned the Government to "open its eyes" to the risk posed by terrorists.
A matter of hours after he was captured on camera, the 22-year-old 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.
Ms Rudd told BBC1's Andrew Marr Show: "It's an ongoing operation, there are 11 people in custody, the operation is still at full tilt, in a way.
"Until the operation is complete, we can't be entirely sure that it is closed."
She said Islamic State, also known as Daesh, was trying to "weaponise" young Britons and defended the work of the security services following claims warnings about Abedi were not followed up.
The murderer had been a former "subject of interest" to MI5 although Ms Rudd said: "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 failure of security services to head off this week's attack was brought into the spotlight by the family of murdered Georgina Callander.
The 18-year-old was killed in the blast shortly after her favourite artist Ariana Grande left the stage at the Manchester Arena on Monday.
In a statement released through Greater Manchester Police, her family said the teenager's life had been cut short by "evil, evil men prepared to ruin lives and destroy families".
It added: "I wish I could say that Georgina is one of the last to die in this way but unless our Government opens its eyes we know we are only another in a long line of parents on a list that continues to grow."