A Durham Police officer has been arrested during an inquiry into the sale of firearms.
The investigation began after a rifle was stolen during a burglary at a farm in County Durham.
It emerged it had earlier been handed to police for destruction by a previous owner, but had then disappeared from the registration system.
The 46-year-old is being questioned on suspicion of malpractice in public office, theft, forgery and deception.
Durham Police Professional Standards Department is handling the investigation, which is believed to involve more than 100 weapons.
Ch Supt Ian Macdonald said the matter first came to light in February.
"We began an investigation immediately," he said.
"As a result, we have established that a large number of weapons, the majority shotguns, handed in to the police by members of the public for disposal or destruction have been sold on.
"We have established where all of these weapons have gone and can confirm that they are in the possession of licensed shotgun or firearm certificate holders, all respectable members of the community."
The arrested officer, a long-serving member of the force, has been suspended from duty.