LONDON (Reuters) - British intelligence experts no longer believe another attack is imminent after police made significant progress in their investigation into a suicide bomb attack on a pop concert in Manchester, Prime Minister Theresa May said on Saturday.
Police arrested two more men as they closed in on a suspected network behind bomber Salman Abedi, a 22-year-old Briton born to Libyan parents, and said they had a greater understanding of how his device was made.
They issued a photograph of Abedi taken on Monday night before he blew himself up killing 22 people and said they believed he had assembled his bomb in an apartment in the city centre.
May said the developments meant the Joint Terrorism Analysis Centre (JTAC), the independent body which sets the threat level, had decided it should be lowered from its highest rating "critical", meaning an attack could be imminent, to "severe".
"A significant amount of police activity has taken place over the last 24 hours and there are now 11 suspects in custody," May said.
"The public should be clear about what this means. A threat level of "severe" means an attack is highly likely. The country should remain vigilant."
The threat assessment has now been returned to the level it was at prior to the attack in Manchester, northwest England, and means soldiers who have been assisting police would be withdrawn from Britain's streets from midnight on Monday.
"In the past five days we have gathered significant information about Abedi, his associates, his finances, the places he had been, how the device was built and the wider conspiracy," Greater Manchester Police Chief Constable Ian Hopkins and Neil Basu, Senior National Coordinator UK Counter Terrorism Policing, said in a joint statement.
British officials have confirmed Adebi had recently returned from Libya and the officers said police needed information about his movements from May 18 when he returned to Britain.
They released a picture taken from closed circuit television showing Adebi before the attack, wearing glasses and a grey baseball cap, carrying a black rucksack and dressed in jeans, trainers, a black hooded top and black body warmer.
"We know one of the last places Abedi went was the city centre flat and from there he left to make his way to the Manchester Arena," their statement said. "The flat is highly relevant as a location which we believe may be the final assembly place for the device."
They said the investigation was not slowing, with some 1,000 officers involved, and the priority was to determine if more people were involved in planning the attack.
As well as killing 22 people, including seven children, Monday's blast injured 116 with 63 still in hospital and 20 in critical care, health officials said.
Security services had feared an experienced bomb-maker could be at large but a source with knowledge of the investigation told Reuters on Thursday Abedi might have made the bomb himself or with an accomplice, lessening the risk of another attack.
"We are getting a greater understanding of the preparation of the bomb," Britain's most senior counter-terrorism officer Mark Rowley said. "There is still much more to do. There will be more arrests."
Rowley, who said on Friday police were confident they had apprehended a "large part of the network", said they had searched or were still examining 17 addresses, mainly in northwest England, and there would be further raids.
Earlier this week a source with knowledge of the matter told Reuters the security services were managing 500 active operations involving some 3,000 people thought to pose a threat.
The Times newspaper said on Saturday that intelligence officers had identified 23,000 jihahist extremists living in Britain.
Despite the lowering of the threat level, extra armed officers were on duty across the country and security has been stepped up at some 1,300 events over a long holiday weekend.
There are a number of high-profile events over the weekend including soccer cup finals in London and Glasgow, and the Great Manchester Run.
A large police presence was in place at the Old Trafford cricket ground in Manchester where some 50,000 people were expected on Saturday for a concert by the Courteeners rock band.
While police and politicians have praised communities in Manchester for their reaction to the bombing, Hopkins said there had been a rise in reported hate crimes, from an average of 28 to 56 incidents on Wednesday.
"We can't directly link these to the events of Monday night and are continuing to monitor the situation," he said.
Political campaigning for the June 8 national election, which was suspended after the Manchester attack, resumed on Friday with the bombing becoming a central feature.
Polls suggested the lead of May's ruling Conservatives over the opposition Labour Party has narrowed since the Manchester attack but she is still on course to win comfortably.
Additional reporting by Phil Noble in Manchester; Editing by David Clarke and Gareth Jones