An explosion was reported in the early hours but the circumstances and whersbouts of it “remain unexplained” according to police.
However army personnel did recover “a number of ageing detonators” during their search of the area.
North Yorkshire Police said the noise of an explosion was reported by staff from Marne Barracks, near Catterick, at 3am.
Bomb disposal experts will carry out a controlled explosion and the A1 will reopen “when it is safe to do so”, police said.
The road was opened shortly after 5.30pm.
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.
“The A1 will reopen when it is safe to do so.
“Please note these are NOT the cause of the original explosion which remains unexplained, but were found during the searches today.”
There have been no other reports of damage in the area and the police search was ongoing all day to locate any scene of the explosion.
Police said their search has been affected by darkness and poor weather conditions.
In an earlier statement, the force said the search remained “focused on the area outside of the barracks and the area adjacent to the A1, although at this stage, it cannot be ruled out that it has occurred within the boundary of the barracks”.
Royal Military Police from the Garrison could be seen in their distinctive red caps assisting police by directing traffic in the village of Catterick and speaking to drivers.
A spokesman for Catterick Garrison said four houses within the garrison but outside the barracks had been evacuated.
Those people have now been able to return to the houses, he added.
On the Ministry of Defence (MoD) 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.
Catterick Garrison provides a base for Headquarters 4th Infantry Brigade and Headquarters North East which forms part of 1st (United Kingdom) Division, according to the MoD website.
There are over 13,000 personnel, military, civilian and their dependants, living and working in the area.
Both slip roads to Catterick have also been closed, diversions are in place and motorists have been advised to avoid the area.
Football fans were expected to be among the motorists affected by the road closure.
Newcastle United said they notified West Ham United officials that travelling supporters would be affected.
The North East football club were playing West Ham United today at the latter’s Upton Park ground in east London.