A spokesman for the Independent Police Complaints Commission (IPCC) said the inquiry was examining 90 cases that have raised concern in the Cleveland Police force area, including an investigation into a death that has been reopened.
Confirming that families affected by the 90 cases in Cleveland had been notified of the inquiry, the IPCC said the investigation began in February 2011 when the scenes-of-crime officer was suspended from duty.
Major Incident Teams at both Staffordshire and Cleveland Police have been working on the investigation, under the direction and control of the IPCC.
In a statement, the IPCC said: "The managed investigation is examining allegations that the man conducted sub-standard work, potentially undermining investigations into a range of incidents including suspicious deaths, and had lied about his qualifications when involved in arson investigations.
"The investigation has so far examined all incidents that the man was involved with and narrowed the focus down to 90 cases where concerns exist in the Cleveland Police force area.
"One investigation into a death in Cleveland has been reopened as a result of the initial findings of the managed investigation."
Beattie worked for Staffordshire Police from 1996 to 2002, Northumbria Police for a short period in 2002 and Cleveland Police from 2002 to 2011, although no concerns have been found relating to his employment with the Northumbria force.
IPCC Commissioner Nicholas Long said: "This has been a long-running and complex investigation involving three police forces and covering a period of 15 years.
"We wanted to ensure we reached a stage where we could inform those families directly affected rather than publicise the investigation earlier and possibly create concern for a larger number of people."
Mr Long added: "The allegations are serious and had the potential to impact a large number of investigations. The number has been narrowed significantly by the work already undertaken.
"As this remains a criminal investigation we do not intend to go into specific detail about cases at this stage."
Cleveland Police said in a statement: "There is a dedicated investigation team which has examined all the incidents the man attended and there are specific cases which have been identified where the force has taken the decision to write to the people concerned to inform them about the situation and answer any questions they may have."