No clear explanation has yet been found for a blast heard in the Waterside area of Derry yesterday, despite extensive searches by police.
The loud bang was reported across the city and as far away as City of Derry Airport, but it was loudest in the Waterside - where it shook buildings and windows.
Theories have been put forward, with gas mains being checked and enquiries made into whether an exploding lorry tyre could have sparked the alert.
Meanwhile, residents who had to leave their homes in the Waterside have been allowed to return.
Cllr Gerard Diver says the police have now narrowed down their search......