© Peter Macdiarmid/Getty ImagesPolice said the loud explosion could be heard all over Derry
Residents in Derry who were forced to flee their homes on Tuesday after reports of a loud explosion were able to return on Wednesday afternoon.
Mystery still surrounds the origin of the blast, which could be heard six miles away at the city's airport.
A security operation in the Spencer Road and Fountain Hill areas on Wednesday morning failed to find any explosive device, although the Police Service of Northern Ireland said further searches would be carried out over the next few days in the districts.
Superintendent Chris Yates said one line of inquiry was that the incident was an attack on police. He said they were investigating reports of masked men being seen in the area around the time of the incident.
Yates said the explosion was heard all over the city on Tuesday.
"It shook houses, shook windows and was heard as far as City of Derry airport.
"There are reports that a lorry blew its tyre at around the same time. It is a possibility, but would that sort of incident cause the shockwaves and noise that was felt? I doubt it.
"There are also reports that a gas cylinder exploded. That is a possibility we need to look into.
"What we also have is reports of masked men who made off from Dunfield Terrace. There are reports of debris landing on top of buildings.
"There were police officers in the area at the time. We are not entirely sure what happened and we are just being very careful."
Could this have just been a military aircraft causing a sonic boom? Or some other high altitude military self destruct mechanism? I understand anything over 50,000 feet to be military airspace so in the daylight not much if anything would be seen at high altitudes.