Detonators have been found after an "unexplained" explosion was heard near the Army’s largest training base, meaning parts of the A1 in Yorkshire were closed to traffic for the whole of Saturday.
Police found "a number of ageing detonators" during a search of the area and bomb disposal experts plan to carry out a controlled explosion.
The road, which was shut for six miles around Catterick Garrison, was due to reopen at 3pm but remained sealed off until it reopened earlier this evening.
There were no reports of any injuries and police said the closure was a "precautionary measure" while investigations are carried out.
In a statement, police said: "With the assistance of the military, officers have recovered a number of ageing detonators from the side of the A1 adjacent to Marne Barracks. These detonators are being removed by the army's bomb disposal team to a safe area where a controlled explosion will take place.
"Residents should not be alarmed should they hear an explosion in the Catterick area within the next hour."
"Please note these are NOT the cause of the original explosion which remains unexplained, but were found during the searches today," the statement added.
As questions surfaced on social media about the reason for the delay, North Yorkshire Police tweeted that safety was their top priority and the force will "not take any chances".
The alarm was reportedly raised by just one caller and police searching an area outside of the barracks, adjacent to the motorway, have so far been unable to find a blast site.
But the report is not being treated as a hoax and officers are appealing for anyone with information on "anything happening at 3am...that may have resulted in a noise similar to that of an explosion".
A spokesperson for North Yorkshire Police did not say where the explosion was or what caused it, only that it was heard in the area at 3am.
Four houses on garrison land outside the barracks were evacuated, a spokesperson for Catterick said, but the families have since been allowed to return to their homes.
Police said there were no reports of damage in the area and the search for the scene of the explosion had been hampered by darkness and poor weather conditions.
Both slip roads to Catterick have also been closed and diversions have been put in place as drivers are avoided to avoid the area.
The closure of around six miles is causing heavy congestion on the road, which runs alongside Marne Barracks.
Catterick Garrison, near Catterick village and Ripon, covers 2,400 acres, according to the Army website, and is the home of several regiments and the Infantry Training Centre.
More than 13,000 personnel and their families live and work in the area.
It was the scene of a deadly helicopter crash in 2007, when an RAF Puma went down during an exercise at the base, killing three people.
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