Police officers with dogs have today (Monday) been conducting a search on and around the verges of the A43 roundabout south of the M1 junction 15a to find lost bullets.
Lanes have been closed throughout the day in order to protect officers and dogs.
The search is now completed and the roads open as normal.
Firearms officers were responding to an emergency call in the early hours of today.
A police spokesman said: “During their response, a box containing bullets which had been left on the roof of their police vehicle fell into the road.
“The box has been recovered and five bullets are currently unaccounted for.
“A thorough search of the area, which includes use of specially trained dogs, has been under way since this morning.
“It is possible that the remaining few bullets could have been carried a distance in the tyres of large vehicles.
“Our priority is the safety of members of the public. If anyone finds or sees anything they suspect to be a bullet they should not touch the item and should immediately call 101.”
Northamptonshire Police takes the security surrounding armed police and their equipment extremely seriously and this incident will be thoroughly investigated.
The incident has been voluntarily referred to the IPCC.