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.
Greater Manchester Police Chief Constable Ian Hopkins said the level of activity in the probe is "intense" and continuing "at pace".
Asked if officers are looking for the person who made the bomb, he said: "I think it's very clear that this is a network that we are investigating and as I've said, it continues at pace, this extensive investigation is going on and activity taking place across Greater Manchester as we speak."
The Manchester bomber is believed to have travelled to Syria and had "proven" links with Islamic State, France's interior minister has said.
Gerard Collomb told French television that both British and French intelligence services had information that Abedi had been in Syria.
Ms Rudd disclosed that the intelligence services had been aware of British-born Abedi.
Asked if she knew whether Islamic State (IS) directed the attack, she said: "We don't have that information yet no."
Ms Rudd said the investigation was "ongoing" when asked if another attack was planned.
She said: "It was a devastating occasion, it was ... more sophisticated than some of the attacks we've seen before and it seems likely, possible that he wasn't doing this on his own, so the intelligence services and the police are pursuing their leads in order to make sure that they get all the information and reduce therefore the risk that they need to keep us all safe."
Speaking on BBC Radio 4's Today programme, she added: "Well the security services will know a lot of people, it doesn't mean that they are expected to arrest everybody they know, but it is somebody who they had known before and I am sure when this investigation concludes, we'll be able to find out more."
She went on: "I have complete confidence in our security services, in our intelligence services and I am determined to make sure that they get the support they need to carry on this investigation to keep us safe."
Here’s what we know so far about him.
Where is he from?
Believed to be of Libyan descent, Abedi was born in Manchester.
Abedi was registered as living at Elsmore Road in the city as recently as last year – where police raided a downstairs red-bricked semi-detached property on Tuesday as they hunted those thought to be behind the blast.
He is thought to have lived at a number of addresses in the area, including one in Wilbraham Road, where plain clothes police made an arrest on Tuesday.
What do we know about him?
It was unclear whether he lived alone at the address, where a flag – possibly Palestinian – was occasionally seen flying from an upstairs window.
Neighbours have recalled an abrasive, tall, skinny young man who was little known in the neighbourhood, and often seen in traditional Islamic clothing.
They said the young man had many visitors, one man dropping by regularly to pick him up in a Toyota Yaris.
Abedi is registered as having lived with his mother Samia Tabbal, father Ramadan Abedi and a brother, Ismail Abedi, who was born in Westminster in 1993.
A childhood friend of Ismail, who asked not to be named, described Salman as “normal” and said his family were known to the Libyan community in the city.
“Ismail’s brother was kind of like a normal guy,” he told the Press Association. “I’ve never chilled with his brother. I know his name is Salman and I say ‘hi’ to him and talk to him.
“He was always friendly, nothing to suggest (he was violent). He was normal, to be honest.”
According to the man, Ismail teaches Arabic classes at a mosque in the area, which his father was also said to visit.
Abedi is also thought to have a younger brother, named as Hashim Abedi, and a sister Jomana, whose Facebook profile suggests she is from Tripoli and lives in Manchester.
The suspect also studied on a business and management course at Salford University two or three years ago, a source said, but dropped out of the course and did not complete his degree. He would have graduated this summer.
What did he do?
Abedi is suspected to be the lone male suicide bomber who detonated an improvised explosive device at Manchester Arena after an Ariana Grande concert. The blast killed 22 people and injured 59 others.
The terror attack has claimed more lives than any other in Britain since the 7/7 attacks in London in 2005, and Islamic State has claimed responsibility for the attack.
Greater Manchester Chief Constable Ian Hopkins said detectives were working to establish whether Abedi was working alone. Meanwhile, a 23-year-old has been arrested in South Manchester in connection with the incident.
