He was due to retire from the force this summer to take up a post as head of security with a major financial institution.
He became an assistant chief constable in December 1989, taking overall control of the Special Branch, which is the lead agency in gathering and co-ordinating intelligence on the IRA and other groupings.
Mr Fitzsimons, 52, had previously been a chief superintendent in day-to-day operational control of the Special Branch since 1985. He had joined the RUC in 1963.
He enjoyed a high reputation both within the branch and the force as a whole - although the branch, in common with military intelligence and MI5 and MI6 - failed to spot the IRA's Libyan connection which led to the re- arming of the republican group in 1985-86.
The terrorists successfully managed to escape detection as they smuggled several large shipments of weaponry and explosives from Tripoli in those years.
According to one source who knew him well, Mr Fitzsimons worked on very easy terms with his counterparts at the head of Military Intelligence.
An exception to this was the lieutenant-colonel who headed Army intelligence in Northern Ireland in the late 1980s and who gave evidence during the trial of the Army agent Brian Nelson.
Mr Fitzsimons reportedly had serious reservations about this officer's character and behaviour. He is understood to have suspected that Nelson had been run as an agent in an improper manner with scant regard to agreed procedures.
The loss of Mr Fitzsimons comes at a time when a number of other very experienced senior RUC officers have just retired from the force.
The Special Branch will be particularly hard-hit by yesterday's crash, since at least seven of the 10 RUC casualties were senior officers in the department. They included, it is believed, a chief superientendent and a superintendent.Reuse content