Cleveland Police Authority meets today to discuss a letter of complaint against senior officers sent by lawyers acting for suspended detective Ray Mallon.

The police force has been the subject of several investigations in recent years.

This is a brief guide to the main events and the key players.

OPERATION LANCET

This was launched in September 1997 following allegations that drugs were being offered for confessions.

The first two officers suspended as part of Operation Lancet were Detective Constables Brendon Whitehead and Sean Allen, in October 1997. Their boss, Detective Superintendent Ray Mallon, was suspended on December 1.

The £5m inquiry, which is being carried out by West Yorkshire Police, is in its final stages and no criminal charges have been brought against any of the suspended officers.

Two officers have been charged with disciplinary offences.

OPERATION EAGLE

This is the disciplinary arm of Operation Lancet. It was headed by Chief Superintendent Kevin Pitt until July this year, when he moved to take up the post of district commander at Stockton.

Cleveland Police said the move had nothing to do with criticism directed at Mr Pitt by a judge following the collapse of a trial involving two Middlesbrough CID officers.

The cost of Eagle is linked to Operation Lancet.

OPERATION DOLLAR

This was triggered by a 200,000-word submission made by Det Supt Mallon.

The investigation, into senior officers at Cleveland Police, was supervised by Detective Chief Superintendent Peter Barclay, of Northamptonshire Police.

It is now complete, and the files are with the Crown Prosecution Service. The cost unknown.

OPERATION DIAMOND

This was set up in August to look into the conduct of Cleveland Chief Constable Barry Shaw.

It followed a complaint made by the force's former head of media affairs, Joanne Malone, who used the Public Interest Disclosure Act to accuse Mr Shaw of authorising the leak to a national newspaper of a confidential report critical of Mr Mallon's zero tolerance style of policing.

The cost of the inquiry, being conducted by the Chief Constable of South Yorkshire, Michael Hedges, is unknown.

OPERATION TERN

The operation was set up to investigate claims made against former assistant chief constable Dave Earnshaw and former assistant chief constable Richard Brunstrom.

Nottinghamshire Police conducted the inquiry and it resulted in Cleveland Police Authority deciding to take no disciplinary action against Mr Earnshaw.

The North Wales Police Authority is still deciding if any action needs to be taken against Mr Brunstrom, who is now assistant chief constable with North Wales Police.

OPERATION TEAK

Launched in 1996, the operation, supervised by the Police Complaints Authority, investigated alleged links between a number of police officers and drugs informer Brian Charrington, from Middlesbrough.

Four officers were served with disciplinary notices and one was suspended. The file is with the Crown Prosecution Service. The cost is unknown