A Police Complaints Authority (PCA) spokesman said more than 30 officers have received around 200 notices of complaint. The investigation focuses on Middlesbrough CID, whose former head, Detective Superintendent Ray Mallon, is among seven officers suspended pending the outcome of the investigation.
PCA member Tony Williams told Channel 4 News that a hard core of notices related to allegations of "extremely serious matters".
Some were on fairly minor procedural matters but nearly three quarters related to "potentially serious criminal matters".
He said: "There are some corruption issues. There are other issues, generally speaking, if one generalises, to do with possible assaults, intimidation and undue pressure on people."
A PCA spokesman said: "There are 40 officers working on the case, led by Assistant Chief Constable Andrew Timson of Northamptonshire police, and it is overseen by the PCA."Reuse content