CCTV photo released of killer on day of Manchester bombing

British police have released a photograph of Salman Abedi on the night he killed 22 people in a suicide bomb attack in Manchester and say they believe an apartment in the city centre was where he put together his device.

"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," Greater Manchester Police Chief Constable Ian Hopkins and Neil Basu, Senior National Coordinator UK Counter Terrorism Policing said in a joint statement.

"The flat is highly relevant as a location which we believe may be the final assembly place for the device."

Hopkins and Basu said they had gathered significant information about Abedi, a 22-year-old Briton born to Libyan parents, how his bomb was built, his associates, finances, the places he had been and the wider conspiracy.

However, police needed any information the public might have about his movements from May 18 when he returned to Britain.

They said the investigation was not slowing, with some 1000 officers involved, and the priority was to determine if more people were involved in planning the attack.

Britain lowered its security threat level to "severe" following significant activity by police investigating the suicide bomb attack on a pop concert in Manchester last Tuesday.

Police said that they had made two further arrests as they closed in on other possible cell members.

- AAP