Police investigating an unexplained explosion near a barracks in Catterick, North Yorkshire, have found a number of "ageing detonators" at the side of the busy A1 motorway.
Bomb disposal experts have been brought in on Saturday evening to carry out controlled explosions on the devices, but insist these were not the cause of the earlier blast, which remains a mystery.
North Yorkshire Police said it received a report of an explosion at 3am on Saturday but there have been no reports of injuries.
A spokesperson for Catterick Garrisson, which houses the Marne Barracks and 13,000 military personnel and their dependents, said four houses were evacuated as officers looked for the cause of the explosion. Those people have now been able to return to their houses.
The A1 road is expected to remain closed until 6pm on Saturday, the Highways Agency says.
Both slip roads to Catterick have also been closed and diversions put in place. Motorists have been advised to avoid the area and the small village of Catterick was said to be "heavily congested" this morning.
The closures affect a six-mile (10km) stretch of the road, which runs adjacent to the barracks. Newcastle United football fans were some of the motorists affected by the disruptions earlier, as they travelled to watch their team play West Ham in Upton Park, east London.
A spokeswoman for North Yorkshire Police said on Saturday morning they do not know how close the explosion was to the barracks. "At the moment it's only a heard explosion," she said.
A spokesman for Catterick Garrison said the inquiry was police-led and did not have any information about how close the explosion was to the barracks.
On the Ministry of Defence website, Catterick Garrison is described as the army's "largest training establishment". It covers 2,400 acres and 20,000 acres of training land.
The Garrison is situated just a few miles from the A1 and includes Marne Barracks near Catterick Village and the outstations of Ripon, Dishforth and Topcliffe.