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.

Salman Abedi, the bomber behind the Manchester suicide bombing, is seen in this image taken from CCTV on the night he committed the attack in this handout photo released to Reuters on May 27, 2017.

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was at prior to the attack in Manchester, northwest England, and means soldiers who have been assisting police would be withdrawn from Britain’s streets from midnight on Monday.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.