Other members of Manchester bomber Salman Abedi's terror network could still be at large, Home Secretary Amber Rudd said as the operation to hunt them continued at "full tilt".
Ms Rudd said "potentially" other members of Abedi's group could still be free and "we can't be entirely sure that it is closed".
The Home Secretary's comments came after police issued CCTV stills of Salman 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, 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."
The Home Secretary 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, but 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."