The blast happened on a disused railway line near Binfield Street in Tipton in the West Midlands at about 13:00 BST.
Army experts are continuing to clear the area to make it safe, and residents have not yet been allowed back into their homes.Speaking to the media near the scene, Assistant Chief Constable Gareth Cann, said the inquiry was launched after a loud bang was heard by residents.
As a result of that our officers attended and arrived very quickly at the scene.
From what the officers were told they started to make a search of the area and because they’d been told about the large bang – people had also reported seeing some smoke – the officers started to discover some debris, including nails, which seemed to have been scattered over a fairly wide area.
As a result of that they started to evacuate people from the immediate area to make sure that they were safe.
Since then the officers have called in a number of specialist resources, including colleagues from the Army bomb disposal teams and other specialist investigators.