Manchester bomber Salman Abedi was known to the intelligence services “up to a point”, Home Secretary Amber Rudd has said.
The Government raised the terrorism threat level to “critical” - the highest possible rating - on Tuesday amid fears another attack is imminent.
Ms Rudd disclosed that the intelligence services had been aware of British-born Abedi, who is reported to have recently returned to the UK from a visit to Libya.
She told Sky News: “We do know that he was known up to a point to the intelligence services”.
Ms Rudd refused to be drawn about the nature of the information held by the intelligence services about Abedi.
“I am sure that we will get more information about him over the next few days and the next few weeks,” she said.
She said the threat level would remain at a heightened state while the investigation into the attack continued.
“It is an ongoing operation which means that the investigation is continuing to find leads,” she said.
“So until we can be reassured that there is no continued activity around this operation, that is entirely safe around this operation, then it is right that we are at this heightened state of alert.
“It is operationally driven, it is intelligence-driven and we must make sure that we allow our counter-terrorist police, our police and our intelligence services to get on and do their job and this helps give them the space to do just that.”
Announcing the raising of the threat level late on Tuesday, Prime Minister Theresa May said a “wider group of individuals” could have been involved in the Manchester Arena blast.
In a sign of the increased threat, the military could be deployed to support armed police officers, Mrs May added during a live televised statement from Downing Street.
Monday night’s attack at a concert by US pop star Ariana Grande left 22 people dead, including an eight-year-old girl, and dozens injured.