Members of Manchester bomber's network could 'potentially' be at large

British police said on Sunday they had arrested a 15th person in connection with the Manchester suicide bombing, which killed 22 people at a concert hall and injured dozens of others.

Police operation remains at 'full tilt,' says Interior Minister Amber Rudd

Police officers keep watch over junior runners competing in the Great Manchester Run in central Manchester on Sunday. (Phil Noble/Reuters)

Members of Manchester suicide bomber Salman Abedi's network are still potentially at large, British Interior Minister Amber Rudd said on Sunday, after the terrorism threat level was lowered because of significant progress in the investigation.

Police said they have arrested a large part of the network behind the bombing, which killed 22 people at a concert hall, and four more men were arrested over the weekend as police continued to close in on the group.

Asked during an interview on BBC television whether some of the group were still at large, Rudd said: "Potentially. It is an ongoing operation."

Greater Manchester Police said on Sunday they had arrested a 25-year-old man and a 19-year-old man on suspicion of terrorism offences, taking the total number of people arrested in connection with the attack to 15.

Prime Minister Theresa May said developments in the investigation into the bombing meant that intelligence experts had decided to lower the threat level from its highest rating "critical," meaning an attack could be imminent, to "severe."

Police on Saturday released these photos of Salman Abedi, taken at an unknown location on the night of the Manchester bombing. (Greater Manchester Police via AP)

Police have issued a photograph of Abedi, a 22-year-old Briton born to Libyan parents, taken on Monday night before he blew himself up and said they believed he had assembled his bomb in an apartment in the city centre.

British officials have confirmed he had recently returned from Libya and the officers said police needed information about his movements from May 18 when he returned to Britain.

Abdedi was known to British security services before the bombing, the government has said, but Rudd declined to comment on exactly what had been known about him.

Media have reported that people who knew Abedi had raised concerns about him and his views as long ago as five years before he carried out Monday's attack.

"The intelligence services are still collecting information about him but I wouldn't rush to conclusions, as you seem to be, that they have somehow missed something," Rudd said.