Extensive daylight searches are being carried out in Londonderry after reports of a blast in the city.
An explosion was heard in the Waterside area of the city at around 5pm on Tuesday.
The blast was heard up to 10 kilometres away and shook houses in the area, and windows as far away as Drumahoe.
It has not yet been revealed what caused the bang, which may have come from the Spencer Road area, however Chief Superintendent Chris Yates said it may have been an attack on officers.
"I don't want to play that up, I don't want to play that down. It's one of the lines of inquiry."
He said detectives are also investigating reports that a gas cylinder which was set on fire exploded and that a lorry tyre exploded in Foyle Street.
"Is that a big enough explosion to be heard at the airport? I don't know," said Chief Superintendent Yates.
He admitted that varied reports made for a "confusing picture".
There were masked men seen running away from around the Dunhill Terrace area at the time, and there were reports of debris landing on the roof of a building in Spencer Road.
Chief Superintendent Chris Yates
"It made the picture fairly confusing from our point of view."
Up to 50 people were evacuated from their homes overnight, and six people, including a heavily pregnant woman, were put up in a city centre hotel.
Chief Superintendent Yates said people were advised to leave their homes because of the "unclear" situation, and for their own security.
"Had we walked away from that last night and a curious child had picked something up this morning that detonated then that would be a nightmare scenario."
Searches were carried out on Tuesday night, but the inclement weather meant made trying to establish the cause of the explosion was difficult and the police helicopter could not fly in the conditions.
However, a thorough search of the Spencer Road area is being carried out by police officers. It is expected to be finished and residents allowed back into their homes by early afternoon.
Gregory Campbell, DUP MLA for East Londonderry, is keen to see what the searches reveal.
"Let's see if this is an attack on the police, and if it is then the wider community must stand up to that.
"There can be no hiding place. All of us, every political representative, every section of the community, must decided is it this way , are we going to go back to the way it was, or are we going to go forward; now which is it?
Everybody's going to have to make their mark and say this is what I believe," he added.
© UTV News