The Independent Police Complaints Commission has appointed Sussex Police to look into allegations raised during a murder inquiry on Teesside.
Cleveland chief constable Sean Price has referred the matter on
Cleveland Police said the so far unspecified claims were made during an ongoing inquiry linked to the murder of Lee King in Middlesbrough in 2000.
That inquiry was ordered amid claims a businessman had been encouraged to lend a known criminal money to buy a gun.
The Commission announced on Friday that the outside force will investigate.
A Cleveland Police spokesman said last month that the allegations had been made during Operation Walden, which is linked to certain aspects of the Lee King murder investigation.
Cleveland Chief Constable Sean Price had referred the matter to the Independent Police Complaints Commission, he said.
The spokesman said it would be a completely separate inquiry to Operation Walden but with potential links to it.
He said Lee King's parents had been visited by a senior police officer to explain the developments.
Detective Superintendent Steve Ash said: "These allegations have to be investigated, and investigated fully for the benefit of all parties involved.
"This will unfortunately cause upset and distress particularly to the family of Lee King.
"We are very sensitive to their feelings and have visited them on a number of occasions. They will be kept fully informed of the developments.
"It is, however, very important that their privacy is respected throughout the inquiry, which is their strong wish.
"This is a search for the truth and to determine whether the allegations made have any basis in fact."